Interference optical switch and variable optical attenuator

ABSTRACT

The present invention discloses an interferometer optical switch that can carry out switching over a broad band and has a high extinction ratio and large fabrication tolerance. The interferometer optical switch employs a phase generating coupler, the phase difference of the output of which has wavelength dependence, as at least one of the optical multi/demultiplexing device included in the interferometer optical switch. A wavelength insensitive interferometer optical switch is implemented by making the sum 2π{φ 1 (λ)+φΔ L (λ)+φ 2 (λ)} constant regardless of the wavelength, where φ 1 (λ) is the phase produced by the first optical multi/demultiplexing device, 2πφΔ L (λ) is the phase difference of the optical delay line with an optical path length difference of ΔL, and 2πφ 2 (λ) is the phase produced by the second optical multi/demultiplexing device.

TECHNICAL FIELD

The present invention relates to an interferometer optical switch and avariable optical attenuator used for optical communication systems andoptical signal processing, and more particularly to an optical waveguidecircuit capable of switching in a broad wavelength band.

BACKGROUND ART

According to the explosive proliferation of the Internet, increases inthe transmission capacity of optical communication systems areproceeding rapidly with the United States playing a central role. Thekey technique for increasing the transmission capacity is the wavelengthdivision multiplexing (WDM) system. The WDM system has the capability oftransmitting plurality of optical signals with different wavelengthsover a single optical fiber, thereby increasing the communicationcapacity markedly. Although optical communication systems now installedare point to point systems that interconnect nodes individually, opticalcross connect systems and optical add/drop multiplexing (OADM) systemswith higher functions are now being developed. The optical cross connectsystem is a system that carries out switching of transmission lines at anode such as a telephone office. On the other hand, the optical add/dropmultiplexing system is a system that extracts a specified wavelengthfrom multiplexed signals to distribute it to another transmission line,or adds a new signal to the specified wavelength to be sent. Besides theability to multi/demultiplex the optical signals, these systems musthave spatial division optical switches that can switch the optical pathswithout converting the optical signals into electrical signals. Thus,the space division optical switch is one of the important components ofa future optical communication network.

The optical switches used for these commercial communication systemsmust have such characteristics as small size, low cost, low powerconsumption and fast controllability. In addition, optical switches arerequired which have transmittance with small wavelength dependence andpolarization dependence, and is switchable over a broad wavelength band.

While research and development of a variety of optical components havebeen conducted, waveguide-type optical components based on opticalwaveguides formed on a substrate is receiving attention as opticalcomponents superior in mass-producibility, integratability andreliability. The waveguide-type optical switches are mass-produced athigh accuracy and at high reproducibility less than or equal to theorder of wavelength by photolithography and microprocessing, and aresuitable for very large systems because basic elements constituting theswitches are small in size. Thus, they are considered to be mostpromising optical switches. In particular, silica-based opticalwaveguides formed on a silicon substrate are low in loss, and have highreliability and extensibility. Thus, they are expected as a component offuture optical communication systems.

As a basic constituent of the conventional waveguide-type opticalswitches, a Mach-Zehnder interferometer is used. The Mach-Zehnderinterferometer has two couplers and arm waveguides connecting the twocouplers. Driving a thin film heater on the arm waveguides enablesswitching.

(First Example of Conventional Technique)

An optical switch based on conventional waveguide-type opticalcomponents is an interferometer optical switch including opticalmulti/demultiplexing devices and an optical delay line. A typicalinterferometer optical switch is a two-input, two-output Mach-Zehnderinterferometer which is used frequently as a basic element of opticalswitches (Reference Document 1: M. Okuno et al., “Low-loss and highextinction ratio silica-based 1×N thermo-optic switches”, OECC/IOOC 2001Conference Incorporating ACOFT, pp. 39-41, 5 Jul. 2001).

FIG. 37 is a plan view showing a conventional Mach-Zehnderinterferometer optical switch. The Mach-Zehnder interferometer opticalswitch comprises two directional couplers 151 and 152, an optical delayline 131 between the two directional couplers 151 and 152, a phaseshifter 141 formed in the optical delay line, input waveguides 101 and102 and output waveguides 103 and 104. For example, 3 dB-directionalcouplers with the power coupling ratio r=0.5 are used as the directionalcouplers 151 and 152, and a thin film heater is used as the phaseshifter 141. As for the optical path length difference ΔL between twooptical waveguides (optical delay line) connecting the two directionalcouplers 151 and 152, it is set in such a manner that ΔL=0.5λs (=0.75μm) or ΔL=0·λs=0, where λs (=1.5 μm) is the signal wavelength. In theoptical delay line of FIG. 37, ΔL is a relative optical path lengthdifference of the upper side waveguide with respect to the lower sidewaveguide, which includes the effective refractive index of thewaveguides. Generally, the element having ΔL set at 0.5λs at the initialstate is called an asymmetric Mach-Zehnder interferometer optical switchwhich is used as a tap switch or gate switch. On the other hand, theelement having ΔL set a zero is called a symmetric Mach-Zehnderinterferometer optical switch which is used as a bifurcation switch.

FIG. 38 is a cross-sectional view taken along a line XXXVIII-XXXVIII ofthe Mach-Zehnder interferometer optical switch as shown in FIG. 37. On asilicon substrate 161, cladding glass layers 164 and 167 of silica-basedglass are formed. In a mid layer of the cladding glass layers 164 and167, a core glass section 165 of silica-based glass is formed, whichconstitutes optical waveguides. In addition, the phase shifter (thinfilm heater) 141 is formed on a surface of the overcladding glass layer167. In other words, the waveguide-type optical components are formed bythe optical waveguides, thin film heater and so on.

Next, the switching operation of the Mach-Zehnder interferometer opticalswitch as shown in FIG. 37 will be described.

First, the operation of the asymmetric switch will be described. Whenthe phase shifter (thin film heater) 141 is in the OFF state, the switchis in the bar state. Accordingly, the optical signal input via the inputwaveguide 101 is output from the output waveguide 103, and the opticalsignal input via the input waveguide 102 is output from the outputwaveguide 104. By supplying power to the thin film heater 141 to varythe optical path length by a half wavelength (0.5λs·k, where k is aninteger other than zero) using the thermooptic effect, the path lengthdifference becomes ΔL+δΔL=0.5λs−0.5λs=0. When the phase shifter (thinfilm heater) 141 is in the ON state, the switch is in the cross state.Thus, the optical signal input via the input waveguide 101 is outputfrom the output waveguide 104, and the optical signal input via theinput waveguide 102 is output from the output waveguide 103. By thusturning on the thin film heater or not, the optical path length of theoptical delay line 131 is varied, thereby being able to carry out theswitching.

Next, the operation of the symmetric switch will be described. When thephase shifter (thin film heater) 141 is in the OFF state, the switch isin the cross state. Thus, the optical signal is output from the crossport (101

104 or 102

103). In the ON state in which the thin film heater 141 is activated,and the optical path length difference is placed at ΔL+δΔL=0.5λs, theswitch is changed to the bar state so that the optical signal is outputfrom the through port (101

103 or 102

104)

In the bifurcation switch using the symmetric type, when the light isinput to the input waveguide 101, the signal is output from the crossport (output waveguide 104) in the OFF state, but not output from thethrough port (output waveguide 103). On the contrary, in the ON state,the signal is output from the through port (output waveguide 103), butnot output from the cross port (output waveguide 104). In this way, thebifurcation switch is configured such that the light is output from thecross port in the initial OFF state, and is switched to the through portby turning on the thin film heater.

In contrast, a tap switch using the asymmetric type carries out theswitching operation opposite to the bifurcation switch. Thus, it outputsthe light from the through port in the initial OFF state, and switchesthe output to the cross port by turning on the thin film heater. A gateswitch using the asymmetric switch uses only the cross port of the tapswitch. Accordingly, the optical signal is not output from the crossport in the initial OFF state, but is output from the cross port in theON state which is brought about by turning on the thin film heater. Boththe symmetric and asymmetric types are used as a basic element of theoptical switch. In particular, the asymmetric type has an advantage thatit is resistant to fabrication error of optical couplers because it canmaintain a high extinction ratio as long as the power coupling ratios ofthe first and second directional couplers 151 and 152 are equal.

These interferometer optical switches are used as a 1×1 type switch, oras a 1×2 type optical switch used for switching from the power system toa standby system when a failure takes place in the optical communicationsystem. In addition, they are not only used alone, but a configurationis also reported in which a Mach-Zehnder interferometer optical switchhas one of its outputs connected in series to another Mach-Zehnderinterferometer optical switch with the same configuration to increasethe extinction ratio (Reference Document 2: T. Goh et al.,“High-extinction ratio and low-loss silica-based 8×8 thermooptic matrixswitch,” IEEE Photonics technology Letters 1998, Vol. 10, pp. 358-360).

Furthermore, to reduce the power consumption during the ON state of thethin film heater, a configuration is reported which has adiabaticgrooves at both ends of the phase shifter (Reference Document 3: S.Sohma et al., “Low switching power silica-based super high deltathermo-optics switch with heat insulating grooves, “Electronics Letters2002, Vol. 38, No. 3, pp. 127-128).

Moreover, combining the foregoing Mach-Zehnder interferometer opticalswitches as the basic components makes it possible to configure M×Nlarge scale optical switches such as an N×N matrix optical switch(Reference Document 4: T. Goh et al., “Low-loss andhigh-extinction-ratio silica-based strictly nonblocking 16×16thermooptic matrix switch,” IEEE Photonics Technology Letters 1998, Vol.10, No. 6, pp. 810-812), a 1×N tap type optical switch (ReferenceDocument 1), a 1×N tree type optical switch (Reference Document 5: T.Watanabe et al., “Silica-based PLC 1×128 thermo-optic switch,” 27thEuropean Conference on Optical Communication 2001, ECOC '01., Vol. 2,pp. 134-135), and an ROADM (Reconfigurable OADM) switch.

(Second Example of Conventional Technique)

FIG. 39 shows a conventional wavelength insensitive switch (WINS). TheWINS has a configuration in which a first basic circuit 190 is connectedto a second basic circuit that has point symmetry with the first basiccircuit 190. Here, the first basic circuit 190 is a wavelengthinsensitive coupler (WINC) including two directional couplers 151 and152, and an optical delay line 134 between the two directional couplers151 and 152.

The power coupling ratio of the directional coupler 151 (154) is r₁=0.8,the power coupling ratio of the directional coupler 152 (153) is r₂=0.3,and the optical path length difference of the optical delay line 134 isΔL₁ (=−ΔL₃)=0.32 μm. The two waveguides between the first basic circuit190 and the second basic circuit having point symmetry with the firstbasic circuit 190 form an optical delay line 135 whose optical pathlength difference is set at ΔL₂=0. Here, the path length differencerepresents a relative path length difference of a first opticalwaveguide (the lower side optical path in FIG. 39) with respect to asecond optical waveguide. On the optical waveguide of the optical delayline 135, a phase shifter (thin film heater) 142 is formed, and theswitching operation is carried out by supplying power to the thin filmheater.

This circuit can be considered as a circuit configured by replacing thedirectional couplers 151 and 152 of the conventional symmetricMach-Zehnder interferometer optical switch (FIG. 37) with WINCs. Sincethe power coupling ratio of the conventional directional coupler haswavelength dependence, the wavelength range is limited in which itfunctions as a 3 dB coupler, that is, a coupler with the power couplingratio of 0.5. The symmetric Mach-Zehnder interferometer optical switchhas a high extinction ratio when the sum of the directional couplers 151and 152 becomes a perfect coupling length. Accordingly, the extinctionratio is deteriorated when the power coupling ratios of the directionalcouplers 151 and 152 are unequal to 0.5. In contrast, since the circuitof FIG. 39 uses the WINCs, it can set the power coupling ratios atapproximately 0.5 regardless of the wavelength. Since the WINS uses theWINCs whose power coupling ratios have small wavelength dependence, itcan carry out switching in a broader wavelength band than that of theconventional symmetric Mach-Zehnder interferometer optical switch. Inpractice, however, it is difficult to maintain the power coupling ratiosof the WINCs at 0.5 over a broad wavelength band because of thefabrication error and the like. Thus, the wavelength characteristics aredeteriorated by deviation of the power coupling ratios.

In view of this, to adjust the power coupling ratios of the two WINCsconstituting the WINS, fine tuning phase shifters (thin film heaters)141 and 143 are formed on the optical delay lines 134 and 136 of theWINCs (FIG. 40). Since the WINS is a symmetric type, it is in the crossstate in the initial state in which the thin film heaters are notdriven, and the signal input via the input waveguide 101 is output fromthe output waveguide 104. In contrast, when the thin film heaters 141,142 and 143 of the optical delay lines are supplied with heating powersto vary the optical path lengths by δΔL₁, δΔL₂ and δΔL₃ using thethermooptic effect, the WINS is switched into the bar state, so theoptical signal input via the input waveguide 101 is output from theoutput waveguide 103 while preventing output from the output waveguide104. Measuring the wavelength dependence of the transmittance results inan extinction ratio higher than 20 dB over a broad wavelength region of1.2-1.7 μm.

(Third Example of Conventional Technique)

The interferometer optical switch carries out the switching operation bysetting the output intensity at 0 or 1. However, setting the outputintensity at an intermediate value between 0 and 1 makes it possible touse it as a variable optical attenuator for attenuating the intensity ofthe optical signal. As an example, differences between theinterferometer type optical switch and variable optical attenuator willbe described by showing wavelength characteristics of a conventionalasymmetric Mach-Zehnder interferometer. FIG. 41A illustrates thewavelength dependence of the transmittance of the asymmetricMach-Zehnder interferometer optical switch described as the firstexample of the conventional technique. The ON state corresponds to theoutput intensity 1, and the OFF state corresponds to the outputintensity 0. The extinction ratio increases as the transmittance in theOFF state decreases. The optical path length difference of the opticaldelay line 131 (FIG. 37) is ΔL=0.5λs in the initial OFF state, and isΔL+δΔL=0.5λs−0.5λs=0 in the ON state.

FIG. 41B illustrates the wavelength dependence of the transmittance whenthe optical transmittance of the conventional variable opticalattenuator at the center wavelength λc is set at −30 dB, −20 dB, and −10dB. The light intensity can be attenuated to a desired value by settingthe transmittance at an appropriate value by varying the optical pathlength difference of the optical delay line 131 with the phase shifter(thin film heater)

(Problems of Conventional Technique)

The conventional interferometer optical switches or variable opticalattenuators, however, have the following problems.

As for the conventional symmetric Mach-Zehnder interferometer opticalswitch described as the first example of the conventional technique,since its extinction ratio becomes high when the sum of the twodirectional couplers equals the complete coupling length, the highextinction ratio is achieved when the power coupling ratios of the twodirectional couplers are 0.5. However, if the power coupling ratio ofthe directional couplers becomes r1=r2=0.4 because of the fabricationerror, for example, the conditions for the high extinction ratio are notsatisfied, thereby deteriorating the extinction ratio markedly. Inaddition, although the power coupling ratio is set precisely at 0.5, itchanges at different wavelength because there is wavelength dependencein the coupling ratio of the directional couplers. Thus, because of thefabrication error and wavelength dependence of the optical couplers, theconventional symmetric Mach-Zehnder interferometer optical switch cannotbe used in a broad band.

As for the asymmetric Mach-Zehnder interferometer optical switch, on theother hand, since its extinction ratio becomes high when the powercoupling ratios of the two directional couplers are equal, highextinction ratio can be maintained even if the power coupling ratio isr1=r2=0.4 because of the fabrication error, for example. Likewise, evenif the power coupling ratios vary in accordance with the wavelengthbecause of the wavelength dependence, it can maintain high extinctionratio. However, to make the Mach-Zehnder interferometer optical switchasymmetric, it is necessary to set the optical path length difference ofthe optical delay line at 0.5λc. Setting the path length difference at afinite value brings about the wavelength dependence in principle, andthe transmittance varies depending on the wavelength. Although theasymmetric type has the advantage of being more tolerant as regards thefabrication error and wavelength dependence of the optical couplers, itis impossible for the conventional technique to set the path lengthdifference at a finite value without causing wavelength dependence.

Accordingly, conventional Mach-Zehnder interferometer optical switcheshave the wavelength dependence illustrated in FIG. 41A. FIG. 41Aillustrates the wavelength band of 1.45-1.65 μm when the signalwavelength λs is set at 1.5 μm. Although the extinction ratio is good atλs, the extinction ratio, which is defined as the difference between thetransmittances in the ON state and OFF state, deteriorates as thewavelength is away from the signal wavelength. If the target value ofthe extinction ratio is equal to or greater than 30 dB, the range inwhich the conventional Mach-Zehnder interferometer optical switch canachieve the target value is about 60 nm around the signal wavelength λs.Accordingly, at the center wavelength 1.55 μm, for example, theextinction ratio deteriorates to about 25 dB. Consequently, theconventional Mach-Zehnder interferometer optical switch is operationalin a limited wavelength range. Hence, it is not suitable for wavelengthdivision multiplexing transmission systems or the like, which requiresoperation over a broad wavelength band.

As for the WINS described as the second example of the conventionaltechnique, since it can reduce the wavelength dependence of the powercoupling ratios of the optical couplers, it can make the wavelengthdependence less than the optical switch of the first example of theconventional technique. However, since the WINS is based on thesymmetric Mach-Zehnder interferometer optical switch, it is necessary toplace the power coupling ratios of the two WINC at 0.5 to obtain a highextinction ratio. Although using the WINCs enables the reduction of thewavelength dependence, it is impossible to maintain the power couplingratios at 0.5 throughout the wavelength band. Accordingly, if the powercoupling ratios become r1=r2=0.45 at a particular wavelength, forexample, the extinction ratio deteriorates greatly. The wavelengthdependence can be improved by forming phase shifters in the opticaldelay line of the WINCs as shown in FIG. 40, to fine tune the pathlength difference. However, it is necessary in this case to drive thethree phase shifters simultaneously. Consequently, several problems areposed: (1) the power consumption for switching becomes several timesgreater than that of the conventional case; (2) the control timeincreases because of an increase in the number of the locations to beadjusted; (3) the control algorithm of the switching operation becomescomplicated; and (4) the amount of electrical wiring becomes severaltimes greater than that of the conventional case because of an increasein the number of the phase shifters. As a result, the characteristicsnecessary for a commercial system such as the low power consumption andfast controllability cannot be satisfied.

As for the variable optical attenuator described as the third example ofthe conventional technique, it can take a desired attenuation only atthe center wavelength.

Although Reference Documents 1-5 are enumerated above as the ReferenceDocuments relevant to the present invention, the following documents areintroduced here as other Reference Documents which describe similarconventional techniques.

Reference Document 6: K. Jinguji et al., “Two-port optical wavelengthcircuits composed of cascaded Mach-Zehnder interferometers withpoint-symmetrical configurations.”, Journal of Lightwave Technology1996, Vol. 14, No. 10, pp. 2301-2310.

Reference Document 7: M. Okuno et al., “Birefringence control of silicawaveguides on Si and its application to a polarization-beamsplitter/switch.”, Journal of Lightwave Technology 1994, Vol. 12, No. 4,pp. 625-633.

Reference Document 8: T. Mizuno et al., “Mach-Zehnder interferometerwith a uniform wavelength period,” Optics Letters 2004, Vol. 29, No. 5,pp. 454-456.

Reference Document 9: EP0382461.

Reference Document 10: Japanese patent publication No. 3175499.

Reference Document 11: Japanese patent publication No. 3041825.

DISCLOSURE OF THE INVENTION

The present invention is implemented to solve the foregoing problems ofthe above-described conventional techniques. Therefore it is an objectof the present invention to provide an interferometer optical switch anda variable optical attenuator capable of switching over a broad band andhaving a high extinction ratio and large fabrication tolerance.

To accomplish the object, the present invention utilizes a phasegenerating coupler, the phase difference of the output of which haswavelength dependence, as at least one of the coupler (opticalmulti/demultiplexing device) constituting a Mach-Zehnder interferometer.Then, the present invention is characterized by setting the sum ofphases produced by the phase generating coupler and arm waveguides atconstant regardless of the wavelength, thereby making the outputintensity of the Mach-Zehnder interferometer insensitive to wavelength.

Here, the phase difference of the output of the opticalmulti/demultiplexing device refers to a phase difference produced by thelight launched from two output ports when a light is input into at leastone of the input ports of an optical multi/demultiplexing device, andwhen light are output from at least two output ports of themulti/demultiplexing device. In addition, when light are launched intoat least two of the input ports of an optical multi/demultiplexingdevice, and when a light is output from at least one of the output portsof the optical multi/demultiplexing device, an optical phase differenceoccurs between the two paths with different input ports. In this case,it is possible to consider that light was launched into one of theoutput ports, and were launched from two of the input ports of themulti/demultiplexing device, so the phase difference of the lightlaunched from the two ports of the multi/demultiplexing device cansimilarly be defined. Thus, a phase generating couler is an opticalcoupler that has the ability to produce a wavelength-dependent phasedifference when light is launched into different input/output ports andis launched from different output/input ports.

More specifically, the interferometer optical switch and variableoptical attenuator in accordance with the present invention can providean interferometer optical switch and a variable optical attenuatorhaving a new function that cannot be achieved by the conventionalinterferometer optical switches by comprising: a first opticalmulti/demultiplexing device; an optical delay line including two opticalwaveguides connected to the first optical multi/demultiplexing device; asecond optical multi/demultiplexing device connected to the opticaldelay line; one or more input waveguides connected to the first opticalmulti/demultiplexing device; one or more output waveguides connected tothe second optical multi/demultiplexing device; and a phase shifterinstalled on the optical delay line, wherein at least one of the firstoptical multi/demultiplexing device and the second opticalmulti/demultiplexing device is a phase generating coupler having a phasedifference of an output of which has wavelength dependence.

In addition, an optical multi/demultiplexing device, the phasedifference of the output of which has wavelength dependence, can beimplemented by providing a characteristic of configuring the phasegenerating coupler by interconnecting optical couplers with an opticaldelay line. Furthermore, any desired phase difference can be produced byappropriately setting the power coupling ratios of the optical couplersand the optical path length difference of the optical delay line.

In addition, the phase generating coupler having a phase generatingfunction without a principle loss can be implemented by providing acharacteristic of configuring the phase generating coupler with N+1optical couplers (N is a natural number) and N optical delay linessandwiched between the adjacent optical couplers. Furthermore, theflexibility of parameter setting increases with an increase of N, whichmakes it possible to increase the degree of approximation between thephase difference of the output of the phase generating coupler and anappropriate phase, and the degree of approximation between the powercoupling ratio of the phase generating coupler and an appropriate powercoupling ratio. Thus, it is possible to provide a phase generatingcoupler capable of generating the accurate phase with ease.

Furthermore, the optical path length difference of the optical delayline can be set at any desired value independent of the wavelength bysetting the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}  (1).of the three phase differences at an appropriate value, where λ isoptical wavelength, 2π₁(λ) is a phase difference between light outputfrom the first optical multi/demultiplexing device, 2πφΔ_(L)(λ) is aphase difference caused by an optical path length difference ΔL of theoptical delay line, and 2πφ₂(λ) is a phase difference of the lightoutput from the second optical multi/demultiplexing device. Thus, it ispossible to make the transmission characteristics of the output,wavelength insensitive thereby being able to provide optical componentssuch as an interferometer optical switch and a variable opticalattenuator that can be used in a broad band.

In addition, the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences can be set at (2m′+1)·π (m′ is an integer), and the powercoupling ratios of the first optical multi/demultiplexing device and ofthe second optical multi/demultiplexing device can made substantiallyequal throughout an entire wavelength region. Thus, it can implement awavelength insensitive asymmetric Mach-Zehnder interferometer opticalswitch that cannot be achieved with conventional technology. Forexample, it is possible to provide a broad band gate switch and tapswitch which have a high extinction ratio over a broad wavelength band,and is resistant to fabrication deviations. Furthermore, it can be usedas a wavelength insensitive variable optical attenuator.

In addition, the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences can be set at 2m′π (m′ is an integer), and the sum of thepower coupling ratios of the first optical multi/demultiplexing deviceand of the second optical multi/demultiplexing device can be madesubstantially unity. Thus, it can be operated as a wavelengthinsensitive symmetric Mach-Zehnder interferometer optical switch, forexample.

In addition, the sum of the phase difference of the output of the firstoptical multi/demultiplexing device and the phase difference of theoutput of the second optical multi/demultiplexing device can be equal toΔL/λ+m/2 (m is an integer).

Furthermore, the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences is set such that output intensity of the optical waveguidecircuit becomes constant for the wavelength λ, where 2πφ₁(λ) is a phasedifference between light output from the first opticalmulti/demultiplexing device, 2πφΔ_(L)(λ) is a phase difference caused bythe optical path length difference ΔL of the optical delay line, and2πφ₂(λ) is a phase difference between light output from the secondoptical multi/demultiplexing device. Thus, it can provide aninterferometer optical switch and a variable optical attenuator havingconstant output intensity over a broad band.

In addition, it can provide an interferometer optical switch capable ofcarrying out switching over a broad wavelength band by providingcharacteristics in which, one of the first optical multi/demultiplexingdevice and the second optical multi/demultiplexing device is an opticalcoupler with a phase difference 2πφ_(c) (constant), and the other is aphase generating coupler that is composed of two optical couplers and anoptical delay line sandwiched between the two optical couplers, andpower coupling ratios of the two optical couplers constituting the phasegenerating coupler, and an optical path length difference of the opticaldelay line are set to satisfyφ(λ)=ΔL/λ+m/2−φ_(c)  (2).

Furthermore, it can provide an interferometer optical switch capable ofcarrying out switching over a broad wavelength band by providingcharacteristics in which, the first optical multi/demultiplexing deviceand the second optical multi/demultiplexing device are each a phasegenerating coupler comprising two optical couplers and a single opticaldelay line sandwiched between the two optical couplers, and that thepower coupling ratios of the two optical couplers and an optical pathlength difference of the single optical delay line constituting each ofthe first optical multi/demultiplexing device and the second opticalmulti/demultiplexing device are set such that the sum of a phasedifference of the output of the first optical multi/demultiplexingdevice and a phase difference of the output of the second opticalmulti/demultiplexing device satisfiesφ₁(λ)+φ₂(λ)=ΔL/λ+m/2  (3).

In addition, it is possible to generate a phase effectively with thephase generating coupler by configuring such that the first opticalmulti/demultiplexing device and the second optical multi/demultiplexingdevice each consists of a phase generating coupler comprising N+1optical couplers (N is a natural number), and N optical delay lines,each of which includes two, first and second, optical waveguides (delaylines) sandwiched between adjacent optical couplers of the N+1 opticalcouplers, and wherein assuming that the sum of optical path lengths ofthe first optical waveguide constituting the N optical delay lines ofthe first optical multi/demultiplexing device is Σl_(1,1), the sum ofoptical path lengths of the second optical waveguide is Σl_(2,1), thesum of optical path lengths of the first optical waveguide constitutingthe N optical delay lines of the second optical multi/demultiplexingdevice is Σl_(1,2), and the sum of optical path lengths of the secondoptical waveguide is Σl_(2,2), the sum of the optical path lengthssatisfy either (Σl_(1,1)>Σl_(2,1) and Σl_(1,2)>Σl_(2,2)), or(Σl_(2,1)>Σl_(1,1) and Σl_(2,2)>Σl_(1,2))

Furthermore, it can be characterized in that the first opticalmulti/demultiplexing device and the second optical multi/demultiplexingdevice each consist of a phase generating coupler including N+1 opticalcouplers (N is a natural number), and N optical delay lines sandwichedbetween adjacent optical couplers of the N+1 optical couplers, and thatthe power coupling ratios of the N+1 optical couplers of the firstoptical multi/demultiplexing device and of the second opticalmulti/demultiplexing device are made equal. This makes it easier tofabricate the optical couplers, and thus, improves the processing yield.

In addition, using a directional coupler consisting of two opticalwaveguides placed side by side in close proximity as the optical couplermakes it possible to set the power coupling ratio of the optical couplerat any desired value by appropriately setting the coupling length of thetwo optical waveguides and the spacing between the waveguides.

Furthermore, using a thin film heater formed on the optical waveguide asthe phase shifter makes it possible to operate the switch with highaccuracy.

In addition, forming adiabatic grooves near the phase shifter enablesthe suppression of the power consumption required for switching.

Furthermore, a low loss optical waveguide circuit superior inintegratability, reliability and stability can be offered by providing acharacteristic of configuring the optical waveguide circuit withsilica-based glass optical waveguides.

In addition, a plurality of interferometer optical switches may beconnected to improve the extinction ratio and to provide aninterferometer optical switch with higher functions. Furthermore, largescale interferometer optical switches such as an N×N matrix switch, a1×N tree type switch, a 1×N tap type switch, an M×N DC switch and anROADM switch can be configured by connecting a plurality ofinterferometer optical switches.

In addition, a 1×2 interferometer optical switch with constant powerconsumption can be implemented by providing the characteristic that afirst interferometer optical switch has a first output waveguide of itstwo output waveguides connected to an input waveguide of a secondinterferometer optical switch, and has its input waveguide used as aninput port of the interferometer optical switch; the secondinterferometer optical switch has its output waveguide used as a firstoutput port of the interferometer optical switch; and the firstinterferometer optical switch has a second output waveguide of its twooutput waveguides used as a second output port of the interferometeroptical switch.

Furthermore, a PI-LOSS (path independent loss) 1×2 interferometeroptical switch can be implemented by providing the characteristic that afirst interferometer optical switch has a first output waveguide of itstwo output waveguides connected to an input waveguide of a secondinterferometer optical switch, has a second output waveguide of its twooutput waveguides connected to an input waveguide of a thirdinterferometer optical switch, and has its input waveguide used as aninput port of the interferometer optical switch; the secondinterferometer optical switch has its output waveguide used as a firstoutput port of the interferometer optical switch; and the thirdinterferometer optical switch has its output waveguide used as a secondoutput port of the interferometer optical switch.

In addition, a large scale interferometer optical switch such as an N×Nmatrix switch, a 1×N tree switch, a 1×N tap switch, an M×N DC switch andan ROADM switch can be offered by providing a characteristic ofconfiguring an M input (M: natural number), N output (N: natural number)optical switch by using at least one interferometer optical switch.

Furthermore, a polarization insensitive or polarization-dependentinterferometer optical switch such as a polarization beam switch can beoffered by providing a characteristic of including a birefringent indexadjustment means on the optical waveguide of the interferometer opticalswitch, or of being subjected to the adjustment of a birefringent index.

In addition, the interferometer optical switch, which carries outswitching between the states in which the optical waveguide circuit hasthe maximum and minimum output intensity, can be functioned as avariable optical attenuator by making the output intensity variable andby setting it at any desired value between the maximum and the minimum.In this case, a broad band variable optical attenuator can be providedwhich has constant output intensity over a broad wavelength band.

Furthermore, an optical module of an optical waveguide circuit can beoffered by providing a characteristic of having a module including theoptical waveguide circuit, and optical fibers that are held in themodule and carry out the input and output of a signal to and from theoptical waveguide circuit. The optical module is applicable to opticalcommunication systems such as an optical cross connect (OXC) system oroptical add/drop multiplexing (OADM) system.

According to the present invention, the Mach-Zehnder interferometeremploys as at least one of the first optical multi/demultiplexing deviceand second optical multi/demultiplexing device a phase generatingcoupler, the phase difference of the output of which has wavelengthdependence. This makes it possible to implement an interferometeroptical switch and a variable optical attenuator with new functions thatcannot be implemented by the conventional technique.

In the Mach-Zehnder interferometer optical switch in accordance with thepresent invention including the phase generating coupler, the lightintensity Pc of the cross port (101

104) is given by the following expression.P _(C)=2R(λ)·[1−R(λ)]·[1+cos{2π{φ_(ΔL)(λ)+Φ(λ)}}]  (4).

Where φ_(ΔL)(λ) is a phase difference caused by the optical path lengthdifferences of the optical delay line of the Mach-Zehnderinterferometer, and Φ(λ) is a phase difference produced by the phasegenerating coupler. For the sake of simplicity, it is assumed that thepower coupling ratios of the first and second opticalmulti/demultiplexing devices are equal, and denoted by R(λ). The lightintensity can be made zero by placing 2π{φ_(ΔL)(λ)+Φ(λ)} at an oddmultiple of π. However, for conventional Mach-Zehnder interferometers,it is impossible to set 2π{φ_(ΔL)(λ)} at a constant value regardless ofthe wavelength because φ_(ΔL)(λ) will be wavelength-dependent wheneverΔL is set at a finite value. In contrast, the present invention makes itpossible for the first time to set the phase difference2π{φ_(ΔL)(λ)+Φ(λ)} at any desired constant value regardless of thewavelength by generating an appropriate phase difference using the phasegenerating coupler. The preset invention can offer an interferometeroptical switch and a variable optical attenuator capable of operatingover a broad wavelength band by appropriately setting the phasedifference Φ(λ) of the output of the phase generating coupler inaccordance with the application of interferometer circuits applied.

In addition, since the present invention can implement an interferometeroptical switch circuit capable of switching over a broad wavelengthband, introducing the circuit as a basic element of an optical switchcan implement a switch for an optical cross connect system or opticaladd/drop multiplexing system operating in any desired wavelength band.This makes it possible to use the components in common and to constructthe system at low cost.

Using the phase generating coupler, the phase difference of the outputof which has wavelength dependence, as at least one of the opticalmulti/demultiplexing devices constituting the interferometer makes itpossible to set the optical path length differences of the optical delayline at a finite value without bringing about the wavelength dependence.Thus, the present invention provides an interferometer optical switchthat has a high extinction ratio in a broad band and has large tolerancefor the fabrication error, and a variable optical attenuator operationalin a broad band, which cannot be implemented by the conventionaltechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a configuration of aninterferometer optical switch of a first embodiment in accordance withthe present invention;

FIG. 2 is a graph illustrating the wavelength dependence of the phaserequired in the first embodiment in accordance with the presentinvention;

FIG. 3 is a graph illustrating the wavelength dependence of thetransmittance in the OFF state of an interferometer optical switch ofthe first embodiment in accordance with the present invention;

FIG. 4 is a schematic diagram of a phase generating coupler used in thefirst embodiment in accordance with the present invention;

FIG. 5 is a graph illustrating the wavelength dependence of the requiredphase and the phase difference produced by the phase generating couplerused in the first embodiment in accordance with the present invention;

FIG. 6 is a graph illustrating the wavelength dependence of the powercoupling ratio of the phase generating coupler used in the firstembodiment in accordance with the present invention;

FIG. 7 is a schematic diagram showing a configuration of aninterferometer optical switch of the first embodiment in accordance withthe present invention;

FIG. 8 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of the firstembodiment in accordance with the present invention;

FIG. 9 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch in a first variationof the first embodiment in accordance with the present invention;

FIG. 10 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch in a second variationof the first embodiment in accordance with the present invention;

FIG. 11 is a schematic diagram showing a configuration of aninterferometer optical switch of a second embodiment in accordance withthe present invention;

FIG. 12 is a graph illustrating the wavelength dependence of therequired phase and the phase difference of the light launched from theoutput ports of the optical couplers in the second embodiment inaccordance with the present invention;

FIG. 13 is a schematic diagram showing a configuration of aninterferometer optical switch of the second embodiment in accordancewith the present invention;

FIG. 14 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of the secondembodiment in accordance with the present invention;

FIG. 15 is a schematic diagram showing a configuration of aninterferometer optical switch of a third embodiment in accordance withthe present invention;

FIG. 16 is a cross-sectional view of the interferometer optical switchof the third embodiment in accordance with the present invention;

FIG. 17 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of the thirdembodiment in accordance with the present invention;

FIG. 18 is a schematic diagram showing a configuration of aninterferometer optical switch of a fourth embodiment in accordance withthe present invention;

FIG. 19 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of the fourthembodiment in accordance with the present invention;

FIG. 20 is a schematic diagram showing a configuration of aninterferometer optical switch of a first variation of the fourthembodiment in accordance with the present invention;

FIG. 21 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of a first variationof the fourth embodiment in accordance with the present invention;

FIG. 22 is a schematic diagram showing a configuration of aninterferometer optical switch of a fifth embodiment in accordance withthe present invention;

FIG. 23 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of the fifthembodiment in accordance with the present invention;

FIG. 24 is a schematic diagram showing a configuration of aninterferometer optical switch of a sixth embodiment in accordance withthe present invention;

FIG. 25 is a schematic diagram of a phase generating coupler used in thesixth embodiment in accordance with the present invention;

FIG. 26 is a graph illustrating the wavelength dependence of thetransmittance of the interferometer optical switch of the sixthembodiment in accordance with the present invention;

FIG. 27 is a schematic diagram showing a configuration of aninterferometer optical switch of a seventh embodiment in accordance withthe present invention;

FIG. 28A is a graph illustrating the wavelength dependence of thetransmittance in an OFF state of the interferometer optical switch ofthe seventh embodiment in accordance with the present invention;

FIG. 28B is a graph illustrating the wavelength dependence of thetransmittance in an ON state of the interferometer optical switch of theseventh embodiment in accordance with the present invention;

FIG. 29 is a schematic diagram showing a configuration of aninterferometer optical switch of an eighth embodiment in accordance withthe present invention;

FIG. 30A is a graph illustrating the wavelength dependence of thetransmittance in an OFF state of the interferometer optical switch ofthe eighth embodiment in accordance with the present invention;

FIG. 30B is a graph illustrating the wavelength dependence of thetransmittance in an ON state of the interferometer optical switch of theeighth embodiment in accordance with the present invention;

FIG. 31 is a schematic diagram showing a configuration of aninterferometer optical switch of a ninth embodiment in accordance withthe present invention;

FIG. 32A is a graph illustrating the wavelength dependence of thetransmittance of the TE mode in the OFF state of the interferometeroptical switch of the ninth embodiment in accordance with the presentinvention;

FIG. 32B is a graph illustrating the wavelength dependence of thetransmittance of the TM mode in the OFF state of the interferometeroptical switch of the ninth embodiment in accordance with the presentinvention;

FIG. 33A is a graph illustrating the wavelength dependence of thetransmittance of the TE mode in the ON state of the interferometeroptical switch of the ninth embodiment in accordance with the presentinvention;

FIG. 33B is a graph illustrating the wavelength dependence of thetransmittance of the TM mode in the ON state of the interferometeroptical switch of the ninth embodiment in accordance with the presentinvention;

FIG. 34A is a schematic diagram showing a configuration of an N×N switchusing the interferometer optical switch in accordance with the presentinvention;

FIG. 34B is a schematic diagram showing a configuration of a 1×N switchusing the interferometer optical switch in accordance with the presentinvention;

FIGS. 35A-35E are schematic diagrams illustrating fabrication process ofan optical waveguide circuit;

FIG. 36 is a schematic diagram of an optical switch module using theinterferometer optical switch in accordance with the present invention;

FIG. 37 is a schematic diagram showing a configuration of a conventionalMach-Zehnder interferometer optical switch;

FIG. 38 is a cross-sectional view of the conventional Mach-Zehnderinterferometer optical switch;

FIG. 39 is a schematic diagram showing a configuration of a conventionalwavelength insensitive switch (WINS);

FIG. 40 is a schematic diagram showing a configuration of a conventionalwavelength insensitive switch (WINS);

FIG. 41A is a graph illustrating the wavelength dependence of thetransmittance of an asymmetric Mach-Zehnder interferometer opticalswitch of a first example of the conventional technique; and

FIG. 41B is a graph illustrating the wavelength dependence of thetransmittance when the optical transmittance at the center wavelength λcis set at −30 dB, −20 dB and −10 dB in a conventional variable opticalattenuator.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described withreference to the accompanying drawings.

Throughout the drawings for describing the embodiments in accordancewith the present invention, portions having the same functions aredesignated by the same reference numerals, and their duplicatedescription will be omitted. In the following embodiments, planaroptical waveguides are used, and an interferometer optical switch and avariable optical attenuator will be described which use silica-basedoptical waveguides formed on a silicon substrate as the opticalwaveguides. This is because the planar optical waveguides are superiornot only in the integratability, but also in increasing the switch scaleand reducing the fabrication cost. In addition, this is because theoptical waveguides with this combination are low loss and stable, andare superior in matching with silica-based optical fibers. However, thepresent invention is not limited to these combinations. Furthermore, thewaveguide-type optical switch will be described by way of example of aMach-Zehnder interferometer type 2×2 basic component which is generallyused. However, the present invention is not limited to these, and isapplicable to other switches alike.

FIRST EMBODIMENT

FIG. 1 shows a configuration of the interferometer optical switch of afirst embodiment in accordance with the present invention.

The interferometer optical switch of the present embodiment includes anoptical multi/demultiplexing device (phase generating coupler) 111, thephase difference of the output of which has wavelength dependence; anoptical multi/demultiplexing device 121; an optical delay line 131between the optical multi/demultiplexing devices 111 and 121; phaseshifters 141 formed in the optical delay line 131; input waveguides 101and 102; and output waveguides 103 and 104.

The transmission characteristics of the Mach-Zehnder interferometer isillustrated in FIG. 41A. it has a high extinction ratio at the signalwavelength λs, but the extinction ratio deteriorates as the wavelengthdeparts from the signal wavelength. It will be possible to maintain ahigh extinction ratio throughout the wavelength region if the entirewavelength region can be made the signal wavelength. The signalwavelength is determined by the phase difference corresponding to theoptical path length difference of the optical delay line. Therefore, ifthe optical delay line can be provided with an appropriatewavelength-dependent phase such that the phase difference is keptconstant regardless of wavelength, it will be possible to make theentire wavelength region the signal wavelength.

This principle will be described in a more detail using mathematicalexpressions. When an optical signal is input via the input waveguide 101of the Mach-Zehnder interferometer (see FIG. 37), the light intensity Pcoutput from the output waveguide 104 is given by the followingexpression.P _(C)=0.5·[1+cos{2πφ_(ΔL)(λ)}]  (5).

Where φΔ_(L)(λ) is the phase difference caused by the optical pathlength difference ΔL of the optical delay line 131, and λ is thewavelength. Here, assume that the following embodiments in accordancewith the present invention use the phase difference represented invalues normalized by 2π. In addition, it is assumed that the powercoupling ratios of the two optical multi/demultiplexing devicesconstituting the Mach-Zehnder interferometer are a constant value of0.5. It is obvious from the foregoing expression (5) that the outputintensity of the conventional Mach-Zehnder interferometer has wavelengthdependence in principle because the phase difference due to the pathlength difference of the optical delay line 131 varies with thewavelength.

If the phase difference due to the optical delay line 131 can be setconstant for any wavelength, the Mach-Zehnder interferometer can be madewavelength insensitive. In view of this, the phase compensation iscarried out by utilizing the phase difference of the light output fromthe optical multi/demultiplexing device 111. Assume that light arelaunched into the first optical multi/demultiplexing device 111 of theMach-Zehnder interferometer; that the phase difference between the lightoutput from the two optical waveguides of the opticalmulti/demultiplexing device 111 is φ₁(λ); that the light are launchedinto the two optical waveguides of the second opticalmulti/demultiplexing device 121 of the Mach-Zehnder interferometer; andthat the phase difference between the light output from one of theoutput ports of the optical multi/demultiplexing device 121 is φ₂(λ),then the foregoing expression (5) can be changed as follows.P _(C)=0.5·[1+cos{2π{φ₁(λ)+φ_(ΔL)(λ)+φ₂(λ)}}]  (6).

Here, placing the sum of the phase differences 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}at constant value regardless of wavelength, the output intensity can bemade wavelength insensitive. This is the operation principle of thewavelength independent optical switch disclosed in the presentinvention.

More specifically, a case will now be described in which the principleof implementing the wavelength insensitiveness in accordance with thepresent invention is applied to the Mach-Zehnder interferometer opticalswitch. To operate as the optical switch, the output intensity must bezero in the OFF state, and one in the ON state. Accordingly, setting thesum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the phase difference at m·π (m is aninteger) enables the switching operation because odd m corresponds tothe OFF state, and even m corresponds to the ON state.

Next, the amount of phase difference that needs to be created by theoptical multi/demultiplexing device 111 to make the sum of the phasedifference at constant value will be derived. Since φΔ_(L)(λ) is givenby −φL/λ, the required phase Ψ(λ) is given by the following expression.Ψ(λ)−ΔL/λ=m/2 (m is an integer)  (7).

Here, FIG. 2 illustrates the wavelength dependence of the required phaseΨ(λ) when m=−1 and ΔL=λc/2 (λc is the center wavelength of thewavelength band equal to 1.55 μm), for example. FIG. 3 illustrates thewavelength dependence of the transmission characteristics of theMach-Zehnder interferometer optical switch when the optical delay line131 is provided with a phase given by equation (7). It is seen that thewavelength-dependent phase difference due to the path length differenceof the optical delay line 131 is compensated for, and that the highextinction ratio is obtained over a broad wavelength region.

As a method of providing the actual Mach-Zehnder interferometer with therequired phase as illustrated in FIG. 2, a case of using an opticalmulti/demultiplexing device, the phase difference of the output of whichhas wavelength dependence, will be described. From now on, such anoptical multi/demultiplexing device is called a phase generating coupler(PGC). As a method of implementing the optical multi/demultiplexingdevice, the phase difference of the output of which has wavelengthdependence, a variety of means are conceivable. For example, a phasegenerating coupler can be constructed by optical couplers and opticaldelay lines. In the present embodiment, the phase generating coupler isrealized by an optical multi/demultiplexing device composed of N+1optical couplers (N is a natural number) and N optical delay lines thatconnects adjacent optical couplers. The advantage of using this type ofan optical multi/demultiplexing device for the phase generating coupleris that both the power coupling ratio and the phase difference producedby this optical multi/demultiplexing device can be set at arbitraryvalues, by setting the power coupling ratios of the N+1 optical couplersand the optial path length differences of the N optical delay lines atappropriate values. In addition, the flexibility of parameter settingincreases with an increase of N, thereby being able to improve thedegree of approximation to the target characteristics. Furthermore, theconfiguration has a characteristic that it has no loss in principle.

The interferometer optical switch of the present embodiment asillustrated in FIG. 1 uses only one phase generating coupler 111. Assumethat light are launched into the phase generating coupler 111; that thephase difference between the outputs of the two optical waveguides ofthe phase generating coupler 111 is φ(λ); that light are launched intothe two optical waveguides of the optical multi/demultiplexing device121; and that the phase difference between the light output from theoptical multi/demultiplexing device 121 is φ_(c) (constant), thewavelength-dependent phase difference produced by the phase generatingcoupler 111 is set as follows.φ(λ)=ΔL/λ+m/2−φ_(c) (m is an integer)  (8).

FIG. 4 shows an example of the phase generating coupler 111. The opticalmulti/demultiplexing device (phase generating coupler) 111 as shown inFIG. 4 includes two directional couplers 151 and 152; a minute opticaldelay line 132 consisting of two optical waveguides interconnecting thetwo directional couplers 151 and 152; input waveguides 101 and 102; andoutput waveguides 103 and 104.

The power coupling ratios of the two directional couplers 151 and 152and the path length difference of the single minute optical delay line152 are obtained by using multiple regression analysis in such a mannerthat the power coupling ratio of the optical multi/demultiplexing device111 becomes about 0.5 at the center wavelength λc=1.55 μm of thewavelength region and that the phase difference of the light launchedfrom the optical multi/demultiplexing device 111 satisfies the foregoingexpression (8).

The foregoing expressions (5) and (6) are derived under the assumptionthat the power coupling ratios of the first and second opticalmulti/demultiplexing devices are equal to a constant 0.5 for the purposeof simplicity. In practice, however, it is necessary to consider thewavelength dependence of the power coupling ratio of the opticalmulti/demultiplexing device. When using the Mach-Zehnder interferometeroptical switch in the cross output OFF state, if the power couplingratios of the first and second optical multi/demultiplexing devices areequal, high extinction ratio can be obtained by carrying out the phasecompensation described above. Thus, the optical multi/demultiplexingdevices are set such that the power coupling ratios of the first andsecond optical multi/demultiplexing devices have nearly the samewavelength dependence.

A light is input via the input waveguide 101 of the designed phasegenerating coupler. In this case, the wavelength-dependent phasedifference φ(λ) between the light output from the output waveguides 103and 104 and the wavelength dependence of the power coupling ratio areshown in FIG. 5 and FIG. 6, respectively. In addition, FIG. 5simultaneously shows the phase Ψ(λ) that needs to be generated by thephase generating coupler, that is, the desired function represented bythe right side of the foregoing expression (8). In FIG. 5 and FIG. 6, ΔLis set at 0.34λc (≈0.53 μm), m is set at −1, and φ_(c) is set at −¼ as anumerical example. It is seen from these figures that the phasegenerating coupler functions as a 3 dB optical multi/demultiplexingdevice with the power coupling ratio of about 0.5, and that the phasedifference φ(λ) of the output is nearly equal to the phase Ψ(λ) requiredfor achieving the wavelength insensitiveness.

FIG. 7 is a plan view showing an interferometer optical switch using thephase generating coupler 111. The power coupling ratios of thedirectional couplers 151 and 152 constituting the phase generatingcoupler 111 were set at r₁=0.3 and r₂=0.7, and the path lengthdifference of the minute optical delay line 131 was set at ΔL₁=0.30λc(≈0.47 μm). In addition, the path length difference of the Mach-Zehnderinterferometer 131 was set at ΔL=0.34λc (≈0.53 μm), and the powercoupling ratio of the directional coupler 153 was set at r₃=0.5. Here,the path length difference represents the relative optical path lengthof the upper optical waveguide with respect to the lower opticalwaveguide. The spacing between the two optical waveguidesinterconnecting the optical multi/demultiplexing device 111 and thedirectional coupler 153 of the interferometer optical switch was made250 μm. As the phase shifter 141, a thin film heater was used and itswidth was set at 40 μm, and length at 4 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75% and the core cross section of the optical waveguides was 6×6 μm².

A chip on which the interferometer optical switch was formed was diced,and its switching characteristics were evaluated. Here, the switchingoperation of a gate switch will be described which uses the fabricatedoptical waveguide circuit as a basic component.

When the phase shifter (thin film heater) 141 is in the OFF state, theswitch is in the bar state. Thus, the optical signal input via the inputwaveguide 101 is output from the output waveguide 103, but not from theoutput waveguide 104. In this state, if the thin film heater 141 isactivated, the optical path length is varied by an amount of halfwavelength of the optical signal (0.5λc·k: where k is an integer otherthan zero) by the thermooptic effect, and the path length differencebecomes ΔL+δΔL=0.34λc−0.5λc=−0.16λc. In this case, the phase shifter(thin film heater) 141 is in the ON state, and the switch is in thecross state. Thus, the optical signal input via the input waveguide 101is output from the output waveguide 104. In other words, whenconsidering 101 as the input port, and 104 as the output port, theoptical signal is not output when the phase shifter is in the OFF state,but is output when the phase shifter is the ON state, which means thatthe switch functions as the gate switch. When considering 102 as theinput port, similar switching operation was confirmed.

Next, FIG. 8 illustrates the wavelength characteristics of the measuredtransmittance. The wavelength dependence of the transmittance of theconventional Mach-Zehnder interferometer optical switch as shown in FIG.37 is also illustrated for comparison.

When the phase shifter (thin film heater) 141 is in the OFF state, theinterferometer optical switch of the present embodiment can achieve ahigh extinction ratio of greater than or equal to 40 dB over a broadwavelength band of 1.45-1.6 μm. When the phase shifter is brought intothe ON state, the interferometer optical switch of the presentembodiment achieves a good insertion loss over broad wavelength band.

Thus, we confirmed that using the principle completely different fromthat of the conventional technique, the interferometer optical switch ofthe present embodiment implements a compact switch that has a highextinction ratio over a wide range and is operational with only onephase shifter. In addition, since it carries out the switching operationin the broad wavelength band, it has large tolerance for the powercoupling ratio error of the optical multi/demultiplexing devices and thepath length difference error of the optical delay line. As a result, thepresent embodiment implements an interferometer optical switch that canmaintain a high extinction ratio even if there is fabrication error.

As described above, the interferometer optical switch described in thepresent embodiment is designed such that a high extinction ratio isobtained in a wavelength range of 1.45-1.65 μm. Besides, a highextinction ratio can be obtained at any wavelength region, for example 1um to 2 um, by providing an appropriate phase with optimum design. Inaddition, an optical multi/demultiplexing device composed of N+1 opticalmulti/demultiplexing devices and N optical delay lines that connectsadjacent optical multi/demultiplexing devices is used as a phasegenerating coupler, which is an optical coupler that is capable ofproducing a wavelength-dependent phase difference. However, it isobvious that other optical multi/demultiplexing devices can also be usedto realize a phase generating coupler. Furthermore, its configuration isnot limited to that described in the present embodiment. For example, aconfiguration is also possible which includes three opticalmulti/demultiplexing devices and two optical delay lines sandwichedbetween the adjacent optical multi/demultiplexing devices.Alternatively, a configuration is possible in which the phase generatingcoupler is constructed by combining different opticalmulti/demultiplexing devices. In addition, the opticalmulti/demultiplexing devices are not limited to the directional couplersthe present embodiment uses, but other types of couplers such asmultimode interferometers can be used. Besides, a plurality of types ofoptical multi/demultiplexing devices can be used such as using adirectional coupler and a multimode interferometer as one of and theother of the optical multi/demultiplexing devices constituting the phasegenerating coupler.

Thus, the phase characteristics can be set considering the wavelengthdependence of the power coupling ratios of the opticalmulti/demultiplexing devices used. In addition, locally varying therefractive index of the optical waveguides enables the adjustment of theoptical path length difference and of the coupling characteristics andphase characteristics of the optical multi/demultiplexing devices.Furthermore, although 101 and 102 are used as the input waveguides inthe present example, the same advantages are obtained by using 103 and104 as the input waveguides, and 101 and 102 as the output waveguides.Besides, although it is designed in such a manner that m becomes −1, mcan be +1 or some other integer.

As described above, the present invention is not limited to theconfiguration described here. For example, considering the entirecircuit as a whole, it can configure the interferometer optical switchcapable of maintaining a high extinction ratio over a broad bandregardless of the type of the waveguides, the geometry of thewaveguides, the material of the waveguides, wavelength band, or the typeof the optical multi/demultiplexing devices. The present invention isimplemented by setting the sum of the phase difference of the outputs ofthe optical multi/demultiplexing devices and the path length differenceof the optical delay line at a constant value in the wavelength regionor the frequency region being used.

(First Variation of First Embodiment)

A first variation of the first embodiment in accordance with the presentinvention uses the same configuration as the interferometer opticalswitch of the first embodiment as shown in FIG. 7.

To meet the conditions that the power coupling ratio of the phasegenerating coupler 111 is about 0.5 at the center wavelength λc=1.55 μmof the wavelength region, and the phase difference between the outputlight satisfies the foregoing expression (8), the power coupling ratiosof the two directional couplers 151 and 152, and the path lengthdifference of a minute optical delay line 132 were obtained bypolynomial approximation. As a result, the power coupling ratios of thedirectional couplers 151 and 152 were set at r₁=0.1 and r₂=0.6,respectively, the path length difference of the minute optical delayline 132 was set at ΔL₁=0.27λc (≈0.38 μm), and the power coupling ratioof the directional coupler 153 was set at r₃=0.5. In addition, the pathlength difference of the Mach-Zehnder interferometer was set atΔL=0.37·λc (≈0.53 μm), and the spacing between the two opticalwaveguides interconnecting the optical multi/demultiplexing device 111and the directional coupler 153 was made 250 μm. Here, the path lengthdifference represents the relative optical path length of the upperoptical waveguide with respect to the lower optical waveguide. As thephase shifter 141, a thin film heater was used and its width was set at40 μm, and length at 4 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was1.5% and the core cross section of the optical waveguides was 4.5×4.5μm². Thus, the present example uses the waveguides with the relativerefractive index higher than that of the conventional waveguides. Thisis because the high relative refractive index of the waveguide canreduce the minimum radius of curvature of the waveguides, and hence candownsize the circuit, although the excess loss such as a fiber couplingloss increases.

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heater 141, thereby forming a two-input, two-output opticalswitch module. Forming the interferometer optical switch of the presentembodiment into a module can facilitate the introduction of the switchto optical communication systems such as optical cross connect systemsand optical add/drop multiplexing systems.

Next, the evaluation was made of the switching characteristics of thefabricated interferometer optical switch module. Here, the switchingoperation will be described in the case where the switch module is usedas the gate switch using the fabricated optical waveguide circuit as thebasic component. When the phase shifter (thin film heater) 141 is in theOFF state, the switch is in the bar state. Thus, the optical signalinput via the input waveguide 101 is output from the output waveguide103, but not from the output waveguide 104. Although not shown in FIG.7, by supplying power to the thin film heater formed on the upper sideoptical waveguide (first optical waveguide) of the two delay lines ofthe optical delay line 131, the optical path length was varied by anamount corresponding to half the wavelength of the optical signal(0.5λc·k: k is an integer other than zero) by the thermooptic effect,and the path length difference became ΔL+δΔL=0.30λc+0.50λc=0.80λc. Inthis case, the switch was in the cross state when the phase shifter(thin film heater) 141 was in the ON state, and hence the optical signalinput via the input waveguide 101 was output from the output waveguide104. Thus, we confirmed that the switch functions as a gate switch.Although the foregoing λ is assumed to have a value +1 in the presentexample, it is obvious that k can take other values.

FIG. 9 illustrates the wavelength characteristics of the measuredtransmittance. The wavelength dependence of the transmittance of theconventional Mach-Zehnder interferometer optical switch as shown in FIG.37 is also illustrated for comparison. When the phase shifter was in theOFF state, the interferometer optical switch of the present embodimentwas able to achieve a high extinction ratio in a broader wavelength bandthan the conventional optical switch. When the phase shifter was broughtinto the ON state, the interferometer optical switch of the presentembodiment achieved a good insertion loss in the broad wavelength band.

(Second Variation of First Embodiment)

A second variation of the first embodiment in accordance with thepresent invention uses the same configuration as the interferometeroptical switch of the first embodiment shown in FIG. 7.

To meet the conditions that the power coupling ratio of the phasegenerating coupler 111 is about 0.45 at the center wavelength λc=1.55 μmof the wavelength region, and the phase difference between the outputlight satisfies the foregoing expression (8), the power coupling ratiosof the two directional couplers 151 and 152, and the path lengthdifference of the minute optical delay line 132 were obtained by leastsquare approximation. As a result, the power coupling ratios of thedirectional couplers 151 and 152 were set at r₁=0.4 and r₂=0.8,respectively, the path length difference of the minute optical delayline 132 was set at ΔL₁=0.30λc (≈0.47 μm), and the power coupling ratioof the directional coupler 153 was set at r₃=0.5. In addition, the pathlength difference of the Mach-Zehnder interferometer was set atΔL=0.32·λc (≈0.50 μm), and the spacing between the two opticalwaveguides interconnecting the optical multi/demultiplexing device 111and the directional coupler 153 was made 250 μm. Here, the path lengthdifference represents the relative optical path length of the upperoptical waveguide with respect to the lower optical waveguide. As thephase shifter, a thin film heater was used and its width was set at 40μm, and length at 4 mm. As for the path length difference of theMach-Zehnder interferometer, it was initially set at ΔL=0 μm, and afterthe circuit was fabricated, permanent local heat processing with a thinfilm heater was carried out to vary the refractive index of thewaveguides, thereby adjusting the optical path length difference toΔL=0.32λc (≈0.50 μm).

Thus, in this invention, the optical path length refers to the effectiveoptical path length of the waveguide, which takes into considerationboth the wavelength-dependent refractive index and the path length ofthe waveguide. Accordingly, the optical path length can be altered byvarying the refractive index of the waveguides even after forming thewaveguides. Consequently, after the interferometer optical switch withthe path length difference of zero has been formed, the optical pathlength difference can be adjusted to the design value by varying therefractive index of the waveguides in the fabrication process. Inaddition, the fabrication error can be removed by using the permanentlocal heat processing using the thin film heater. In other words, evenif the optical path length difference deviates from the design valuebecause of the fabrication error, the path length difference can becorrected to the design value by adjusting the refractive index afterthe fabrication. Incidentally, the reason why the present embodimentuses the thin film heater is that the thin film heater has already beenformed on the optical waveguide as the phase shifter. Besides, the thinfilm heater formed on the optical waveguide enables the refractive indexto be adjusted simply and accurately. It is needless to say that theadjusting method of the refractive index is not limited to the thin filmheater, but other means such as light irradiation with a laser can alsobe used. In addition, although the present embodiment uses the thin filmformed for the switching operation to perform the local heat treatment,another thin film heater can be installed to be used specifically forthe local heat treatment to adjust the refractive index. Furthermore,the characteristics of the optical multi/demultiplexing device 111 canbe corrected by adjusting the refractive index of the optical waveguidesof the directional couplers 151 and 152 or of the minute optical delayline 132 constituting the optical multi/demultiplexing device 111.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75%, and the core cross section of the optical waveguides was 6×6 μm².

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,dispersion shifted fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heater 141, thereby forming a two-input, two-output opticalswitch module.

Next, the evaluation was made of the switching characteristics of theinterferometer optical switch module fabricated as described above.Here, the switching operation will be described in the case where theswitch is used as the gate switch employing the fabricated opticalwaveguide circuit as the basic component. When the phase shifter (thinfilm heater) 141 is in the OFF state, the switch is in the bar state.Thus, the optical signal input via the input waveguide 101 is outputfrom the output waveguide 103, but not from the output waveguide 104.Here, by supplying power to the thin film heater 141, the optical pathlength was varied by an amount corresponding to half the wavelength ofthe optical signal (0.5λc·k: k is an integer other than zero) by thethermooptic effect, and the path length difference becameΔL+δΔL=0.32λc−0.50λc=−0.18λc. In this case, the switch was in the crossstate when the phase shifter (thin film heater) 141 was in the ON state,and hence the optical signal input via the input waveguide 101 wasoutput from the output waveguide 104. Thus, we confirmed that the switchfunctions as a gate switch.

FIG. 10 illustrates the wavelength characteristics of the measuredtransmittance. The wavelength dependence of the transmittance of theconventional Mach-Zehnder interferometer optical switch as shown in FIG.37 is also illustrated for comparison. When the phase shifter was in theOFF state, the interferometer optical switch of the present embodimentachieved an extinction ratio greater than or equal to 30 dB over a broadwavelength band of 1.45-1.63 μm. When the phase shifter was brought intothe ON state, the interferometer optical switch of the presentembodiment achieved a good insertion loss over the broad wavelengthband. Although the present example is designed such that the powercoupling ratio (0.45) of the first optical multi/demultiplexing device(phase generating coupler 111) and the power coupling ratio (0.5) of thesecond optical multi/demultiplexing device (directional coupler 153)differ from each other, it can achieve a high extinction ratio over abroader wavelength band than the conventional optical switch. Thus, thefirst and second optical multi/demultiplexing devices can have differentpower coupling ratios, and the power coupling ratios can have differentwavelength dependence.

SECOND EMBODIMENT

FIG. 11 shows a configuration of the interferometer optical switch of asecond embodiment in accordance with the present invention. The circuitof the interferometer optical switch includes a pair of opticalmulti/demultiplexing devices (phase generating couplers) 111 and 112,the phase differences of the outputs of which have wavelengthdependence; an optical delay line 131 between the opticalmulti/demultiplexing devices 111 and 112; phase shifters (thin filmheaters) 141 formed in the optical delay line 131; input waveguides 101and 102; and output waveguides 103 and 104.

In the present embodiment, a configuration will be described whichincludes a plurality of phase generating couplers. Assume that a lightis input to the phase generating coupler 111 in the first stage, and thephase difference between light output from the two optical waveguidesconnected to the phase generating coupler 111 is φ₁(λ); and that lightare launched into the two optical waveguides connected to the phasegenerating coupler 112 at the second stage, and the phase differencebetween the light output from the phase generating coupler 112 is φ₂(λ).Then, the wavelength-dependent phase differences of the outputs of thephase generating couplers 111 and 112 is set to satisfy the followingexpression.φ₁(λ)+φ₂(λ)=ΔL/λ+m/2  (9).Where m is an integer.

Here, the optical multi/demultiplexing device (phase generating coupler)as shown in FIG. 4 is used as the phase generating couplers 111 and 112.The optical multi/demultiplexing device (phase generating coupler) asshown in FIG. 4 includes two directional couplers 151 and 152; a minuteoptical delay line 132 consisting of the two optical waveguidesinterconnecting the two directional couplers 151 and 152; inputwaveguides 101 and 102; and output waveguides 103 and 104. To meet theconditions that the power coupling ratios of the opticalmulti/demultiplexing devices become about 0.5 at the center wavelengthλc=1.55 μm of the wavelength region, and the phase difference betweenthe output light satisfies the foregoing expression (9), the powercoupling ratios of the two directional couplers 151 and 152 constitutingthe phase generating couplers, and the path length difference of theminute optical delay line 132 were obtained by least squareapproximation.

FIG. 12 illustrates the sum of the phase differences of the phasegenerating couplers 111 and 112 thus designed. At the same time, therequired phase Ψ(λ) to be corrected by the phase generating couplers,that is, the desired function given by the right side of the foregoingexpressions (9) is drawn. In FIG. 12, ΔL is set at 0.16λc (≈0.25 μm),and m is set at −1 as a numerical example. It is seen from FIG. 12 thatthe two phase generating couplers each function as a 3 dB opticalmulti/demultiplexing device with the power coupling ratio of about 0.5,and that the sum of the phase differences φ₁(λ)+φ₂(λ) is nearly equal tothe required phase Ψ(λ) required for achieving the wavelengthinsensitiveness.

FIG. 13 is a plan view showing an actually fabricated interferometeroptical switch. The power coupling ratios of the directional couplers151 and 152 constituting the phase generating coupler 111 were set atr₁=0.4 and r₂=0.1. As for the minute optical delay line 132 having twooptical delay lines, a first optical waveguide and a second opticalwaveguide, their optical path lengths are set at l₁₁=502.32 μm andl₂₁=501.99 μm so that the optical path length difference between them isΔL₁=l₁₁−l₂₁=0.21λc (=0.33 μm). Likewise, the power coupling ratios ofthe directional couplers 153 and 154 constituting the other phasegenerating coupler 112 are set at r₃=0.2 and r₄=0.3. As for the minuteoptical delay line 133 having two optical delay lines, a first opticalwaveguide and a second optical waveguide, their optical path lengths areset at l₁₂=463.94 μm and l₂₂=463.68 μm so that the optical path lengthdifference between them is ΔL₂=l₁₂−l₂₂=0.17λc (=0.26 μm). In addition,the present embodiment employs two phase generating couplers, and theiroptical delay lines are disposed in such a manner that the optical delaylines having a greater sum of the optical path lengths are placeddisproportionately on one side (upper side of FIG. 13). Morespecifically, since Σl_(1,1)=l₁₁, Σl_(2,1)=l₂₁, Σl_(1,2)=l₁₂, andΣl_(2,2)=l₂₂, they satisfy Σl_(1,1)>Σl_(2,1) and Σl_(1,2)>Σl_(2,2). Inaddition, the path length difference of the Mach-Zehnder interferometeris set at ΔL=0.16λc (≈0.25 μm), and the spacing between the two opticalwaveguides interconnecting the optical multi/demultiplexing devices 111and 112 was made 200 μm. As the phase shifter 141, a thin film heaterwas used and its width was set at 40 μm, and length at 4 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was1.5%, and the core cross section of the optical waveguides was 4.5×4.5μm².

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heater 141, thereby forming a two-input, two-output opticalswitch module. Then, the switching characteristics of the interferometeroptical switch module were evaluated.

Here, the switching operation will be described in the case where theswitch module is used as the gate switch using the fabricated opticalwaveguide circuit as the basic component. When the phase shifter (thinfilm heater) 141 is in the OFF state, the switch is in the bar state.Thus, the optical signal input via the input waveguide 101 is outputfrom the output waveguide 103, but not from the output waveguide 104.Here, by supplying power to the thin film heater 141, the optical pathlength was varied by an amount corresponding to half the wavelength ofthe optical signal (0.5λc·k: k is an integer other than zero) by thethermooptic effect, and the path length difference becameΔL+δΔL=0.16λc−0.5λc=−0.34λc. In this case, the switch was in the crossstate when the phase shifter (thin film heater) 141 was in the ON state,and hence the optical signal input via the input waveguide 101 wasoutput from the output waveguide 104. In other words, when considering101 as the input port, and 104 as the output port, the optical signal isnot output when the phase shifter is in the OFF state, but is outputwhen the phase shifter is the ON state, which means that the switchfunctions as the gate switch. When considering the 102 as the inputport, similar switching operation was confirmed.

Next, FIG. 14 illustrates the wavelength characteristics of the measuredtransmittance of the interferometer optical switch of the presentembodiment. The wavelength dependence of the transmittance of theconventional Mach-Zehnder interferometer optical switch as shown in FIG.37 is also illustrated for comparison. When the phase shifter 141 is inthe OFF state, the interferometer optical switch of the presentembodiment can achieve a high extinction ratio equal to or greater than40 dB over a broad wavelength band of 1.45-1.6 μm. When the phaseshifter is brought into the ON state, the interferometer optical switchof the present embodiment achieves a good insertion loss over a broadwavelength band.

Thus, the interferometer optical switch described in the presentembodiment uses a novel operation principle to implement high extinctionratio over a wide wavelength region. It was confirmed that the switch isoperational with only one phase shifter. In addition, the switch has alarger tolerance as regards power coupling ratio variations of theoptical multi/demultiplexing device and path length variations of theoptical delay line, since it is operational over a wide wavelengthrange. Accordingly, the present embodiment implements an interferometeroptical switch that can maintain a high extinction ratio even if thereis fabrication error.

Since the present embodiment employs two different phase generatingcouplers, it can increase the phase compensation amount and the degreeof approximation of the power coupling ratios of the opticalmulti/demultiplexing devices, thereby being able to achievecharacteristics better than that of the first embodiment. In addition,an ideal Mach-Zehnder interferometer optical switch is implemented whenthe power coupling ratios of the first and second opticalmulti/demultiplexing devices are 0.5 regardless of the wavelength. Sincethe present embodiment is configured such that it can set the phasedifferences and the power coupling ratios of the first and secondoptical multi/demultiplexing devices without restraint, it can implementan ideal interferometer optical switch.

As described above, the interferometer optical switch described in thepresent embodiment is designed such that it can achieve a highextinction ratio in a wavelength band of 1.45-1.65 μm. However, thepresent invention is not limited to this wavelength region. The switchcan achive a high extinction ratio at any wavelength region, for example1 μm-2 μm, by providing an appropriate phase with phase generatingcoupler. Furthermore, an optical multi/demultiplexing device composed ofN+1 optical multi/demultiplexing devices and N optical delay lines thatconnects adjacent optical multi/demultiplexing devices is used as aphase generating coupler, which is an optical coupler that is capable ofproducing a wavelength-dependent phase difference. However, it isobvious that optical multi/demultiplexing device with otherconfiguration can be used as the phase generating coupler. Besides, theconfiguration is not limited to that described in the presentembodiment. For example, the optical multi/demultiplexing device that isused as the phase generating coupler can be configured by four opticalmulti/demultiplexing devices and three optical delay lines sandwichedbetween the adjacent optical multi/demultiplexing devices, or can beconfigured by combining different optical multi/demultiplexing devices.Furthermore, as for the optical multi/demultiplexing devices used in thepresent embodiment to construct the optical multi/demultiplexing devicethat generates a wavelength-dependent phase difference, they are notlimited to the directional couplers, but other types can also be used.In addition, the phase characteristics can be set considering thewavelength dependence of the power coupling ratios of the opticalmulti/demultiplexing devices used. Besides, it is possible, in thepresent embodiment, to adjust the optical path length difference, andthe coupling characteristics and phase characteristics of the opticalmulti/demultiplexing device by locally varying the refractive index ofthe optical waveguides. ggg, although the present example employs thewaveguides 101 and 102 as the input waveguides, it can achieve the sameadvantages by using the waveguides 103 and 104 as the input waveguides,and 101 and 102 as the output waveguides. Finally, although the presentexample is designed such that m in the foregoing expression (9) becomes−1, m may be +1 or any other integer.

As described above, the present invention is not limited to theconfiguration described here, but can configure the interferometeroptical switch that can maintain a high extinction ratio over a broadband regardless of the types of the waveguides, the geometry of thewaveguides, the material of the waveguides, wavelength band or the typesof the optical multi/demultiplexing device by making the sum of thephase differences of the outputs of the optical multi/demultiplexingdevices and the phase difference due to the path length difference ofthe optical delay line constituting the circuit a constant value in theentire wavelength or frequency band considering the circuit in itsentirety.

THIRD EMBODIMENT

FIG. 15 shows a configuration of the interferometer optical switch of athird embodiment in accordance with the present invention. The circuitof the interferometer optical switch of the present embodiment includesan optical multi/demultiplexing device (phase generating coupler) 111,the phase difference of the output of which has wavelength dependence; adirectional coupler 153; an optical delay line 131 between the opticalmulti/demultiplexing device 111 and the directional coupler 153; phaseshifters 141 formed in the optical delay line 131; input waveguides 101and 102; and output waveguides 103 and 104. As the phase generatingcoupler 111, the present embodiment uses an optical multi/demultiplexingdevice that includes two directional couplers 151 and 152, and a minuteoptical delay line 132 composed of two optical waveguidesinterconnecting the two directional couplers 151 and 152. In addition,three adiabatic grooves 168 are formed at the sides of the pair of phaseshifters 141 on a substrate.

FIG. 16 shows the cross sectional structure taken along the line XVI-XVIof the interferometer optical switch as shown in FIG. 15. On a siliconsubstrate 161, cladding glass layers 164 and 167 composed ofsilica-based glass are stacked. As a mid layer of the cladding glasslayers 164 and 167, a core glass section 165 made from the silica-basedglass is disposed, which constitutes the optical waveguides. Inaddition, on a surface of the overcladding glass layer 167, the phaseshifters (thin film heaters) 141 are formed, and at both sides of thephase shifters 141, the adiabatic grooves 168 are formed. The adiabaticgrooves 168 are located at such position that equalizes the stress nearcore waveguides.

To meet the conditions that the power coupling ratio of the phasegenerating coupler 111 is about 0.5 at the center wavelength λc=1.55 μmof the wavelength region, and the phase difference between the outputlight satisfies the foregoing expression (8), the power coupling ratiosof the two directional couplers 151 and 152, and the path lengthdifference of the minute optical delay line 132 were obtained bypolynomial approximation. As a result, the power coupling ratios of thedirectional couplers 151 and 152 were set at r₁=0.1 and r₂=0.6,respectively, the path length difference of the minute optical delayline 132 was set at ΔL₁=0.27·λc (≈0.38 μm), and the power coupling ratioof the directional coupler 153 was set at r₃=0.5. In addition, the pathlength difference of the Mach-Zehnder interferometer was set atΔL=0.37·λc (≈0.53 μm), and the spacing between the two opticalwaveguides interconnecting the optical multi/demultiplexing device 111and the directional coupler 153 was made 100 μm. Here, the path lengthdifference represents a relative optical path length of the upperoptical waveguide with respect to the lower optical waveguide. As thephase shifters 141, a thin film heater was used and its width was set at40 μm, and length at 4 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75%, the core cross section of the optical waveguides was 6×6 μm², andthe width and the depth of the adiabatic grooves 168 were 70 μm and 35μm, respectively.

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module. Then, the switching characteristics of the interferometeroptical switch module were evaluated.

Here, the switching operation will be described in the case where theswitch module is used as the gate switch using the fabricated opticalwaveguide circuit as the basic component. When the phase shifters (thinfilm heaters) 141 are in the OFF state, the switch is in the bar state.Thus, the optical signal input via the input waveguide 101 is outputfrom the output waveguide 103, but not from the output waveguide 104. Bysupplying power to the thin film heaters 141, the optical path lengthwas varied by an amount corresponding to half the wavelength of theoptical signal (0.5λc·k: k is an integer other than zero) by thethermooptic effect, and the path length difference becameΔL+δΔL=0.37λc+0.50λc=−0.13λc. In this case, the switch was in the crossstate when the phase shifters (thin film heaters) 141 were in the ONstate, and hence the optical signal input via the input waveguide 101was output from the output waveguide 104. In other words, when using thewaveguide 101 as the input port and 104 as the output port, the opticalsignal was not output when the phase shifters 141 were in the OFF state,but was output when the phase shifters 141 were in the ON state, whichmeans that the switch functions as a gate switch. We confirmed the sameswitching operation when using the waveguide 102 as the input port. Inaddition, although the present example employs the waveguides 101 and102 as the input waveguides, it can achieve the same advantages by usingthe waveguides 103 and 104 as the input waveguides, and 101 and 102 asthe output waveguides. Besides, since the optical switch of the presentembodiment has the adiabatic groove structure, it can suppress the powerconsumption of the phase shifters required for the switching to 1/10that of the conventional switch.

Next, FIG. 17 illustrates the wavelength dependence of the transmittancemeasured for the circuit of the present embodiment. The optical switchof the present embodiment can also achieve a high extinction ratio equalto or greater than 30 dB over a broad wavelength band of 1.3-1.6 μm whenthe phase shifters are in the OFF state.

Thus, the interferometer optical switch described in the presentembodiment uses a novel operation principle to implement high extinctionratio over a wide wavelength region. It was confirmed that the switch isoperational with only one phase shifter. In addition, the switch has alarger tolerance as regards power coupling ratio variations of theoptical multi/demultiplexing device and path length variations of theoptical delay line, since it is operational over a wide wavelengthrange. Accordingly, the present embodiment implements an interferometeroptical switch that can maintain a high extinction ratio even if thereis fabrication error. In addition, since the interferometer opticalswitch of the present embodiment has the adiabatic groove structure onthe substrate, it can greatly reduce the switching power. It is obviousthat the geometry and position of the grooves are not limited, and thegrooves can include materials other than air, and that a structure otherthan the optical circuit can be formed on the planar substrate asillustrated in the present embodiment.

FOURTH EMBODIMENT

FIG. 18 shows a configuration of the interferometer optical switch of afourth embodiment in accordance with the present invention. The circuitof the interferometer optical switch of the present embodiment is anoptical switch with a multiple-stage interferometer configuration usinga plurality of interferometer optical switches. Since the multiple-stageinterferometer configuration can block light with plurality of basiccomponents when the switch is in the OFF state, it can achieve anextinction ratio higher than that of the single basic component.

The present circuit is configured by connecting two interferometeroptical switches of the first embodiment as shown in FIG. 7. A firststage (input side) interferometer optical switch 170 has its firstoutput (corresponding to 104 of FIG. 7) connected to a first input(corresponding to 102 of FIG. 7) of a second stage (output side)interferometer optical switch 171, and has its second output(corresponding to 103 of FIG. 7) used as the output waveguide 103. Thesecond stage interferometer optical switch 171 has its second input(corresponding to 101 of FIG. 7) used as the input waveguide 101. Theinput waveguide 101 and the output waveguide 103 intersect with eachother on the way, thereby forming a cross waveguide 155. It is obviousthat such a circuit layout is also possible in which the input waveguideand output waveguide do not intersect with each other. In addition, theinterferometer optical switch 170 of the first stage has its secondinput (corresponding to 101 of FIG. 7) used as the input waveguide 102,and the interferometer optical switch 171 of the second stage has itssecond output (corresponding to 103 of FIG. 7) used as the outputwaveguide 104.

To meet the conditions that the power coupling ratios of the phasegenerating couplers are about 0.5 at the center wavelength λc=1.55 μm ofthe wavelength region, and the phase difference between the output lightsatisfies the foregoing expression (8), the power coupling ratios of thetwo directional couplers 151 and 152, and the path length difference ofthe minute optical delay line 132 were obtained by the conjugategradient method. As a result, the power coupling ratios of thedirectional couplers 151 and 152 were set at r₁=0.3 and r₂=0.7,respectively, the path length difference of the minute optical delayline 132 was set at ΔL₁=0.30·λc (≈0.47 μm), and the power coupling ratioof the directional coupler 153 was set at r₃=0.5. In addition, the pathlength difference of the Mach-Zehnder interferometer was set atΔL=0.34·λc (≈0.53 μm), and the spacing between the two opticalwaveguides interconnecting the optical multi/demultiplexing device 111and the directional coupler 153 was made 100 μm. Here, the path lengthdifference represents the relative optical path length of the upperoptical waveguide with respect to the lower optical waveguide. As thephase shifters 141, a thin film heater was used and its width was set at40 μm, and length at 4 mm. The path length difference of theMach-Zehnder interferometer was initially set at ΔL=0 μm, and after thecircuit was fabricated, permanent local heat processing using the thinfilm heaters 141 was carried out to adjust the optical path lengthdifference to ΔL=0.34λc (≈0.53 μm).

Although the present embodiment forms two thin film heaters 141 on eachof the pair of the optical waveguides constituting the optical delayline 131, and uses one for the local heat processing and the other forthe switching operation, it is also possible to use both for the localheat processing, or for the switching operation. It is obvious thatthree or more thin film heaters can be formed. In addition, the geometryof the thin film heaters 141 is not limited, and the plurality of thinfilm heaters can have different geometry. Furthermore, the thin filmheaters 141 of the pair of the optical waveguides constituting theoptical delay line 131 can be used simultaneously to carry out the localheat processing or the switching operation.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75%, the core cross section of the optical waveguides was 4.5×4.5 μm²,and the width and the depth of the adiabatic grooves were 70 μm and 50μm, respectively.

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,dispersion shifted fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module. Then, the switching characteristics of the interferometeroptical switch module were evaluated.

When the phase shifters (thin film heaters) 141 are in the OFF state,the switch is in the bar state. Thus, the optical signal input via theinput waveguide 102 is output from the output waveguide 103, but notfrom the output waveguide 104. In this case, since the two basiccomponents 170 and 171 block light from launching out of the outputwaveguide 104, the present embodiment can achieve a higher extinctionratio. By supplying power to the thin film heaters 141 of theinterferometer optical switches 170 and 171, the optical path length wasvaried by an amount corresponding to half the wavelength of the opticalsignal (0.5λc·k: k is an integer other than zero) by the thermoopticeffect, and the path length difference becameΔL+δΔL=0.34λc−0.50λc=−0.16λc. In this case, the switch was in the crossstate when the phase shifters (thin film heaters) 141 were in the ONstate, and hence the optical signal input via the input waveguide 102was output from the output waveguide 104. In other words, when using thewaveguide 101 as the input port and 104 as the output port, the opticalsignal was not output when the phase shifters 141 were in the OFF state,but was output when the phase shifters 141 were in the ON state, whichmeans that the switch functions as a gate switch. In addition, althoughthe present example employs the waveguides 101 and 102 as the inputwaveguides, it can achieve the same advantages by using the waveguides103 and 104 as the input waveguides, and 101 and 102 as the outputwaveguides. Besides, since the optical switch of the present embodimenthas the adiabatic groove structure, it can suppress the powerconsumption of the phase shifters required for the switching to 1/10 ofthe conventional switch.

Next, FIG. 19 illustrates the wavelength characteristics of the measuredtransmittance of the interferometer optical switch of the presentembodiment. The wavelength dependence of the transmittance of theconventional Mach-Zehnder interferometer optical switch as shown in FIG.37 is also illustrated for comparison. When the phase shifters 141 arein the OFF state, the interferometer optical switch of the presentembodiment can achieve a high extinction ratio equal to or greater than60 dB over a broad wavelength band of 1.45-1.63 μm because of themultiple-stage interferometer configuration. When the phase shifters arebrought into the ON state, the interferometer optical switch of thepresent embodiment achieves a good insertion loss over a broadwavelength band.

As described above, the plurality of interferometer optical switches inaccordance with the present invention, which are configured in amultiple stage, enable them to function as a single interferometeroptical switch. Although the present embodiment constructs the two-stageinterferometer configuration by combining the two identicalinterferometer optical switches, it is obvious that the twointerferometer optical switches can use different design values. Inaddition, it is also possible to assume a configuration other than thetwo-stage interferometer configuration described in the presentembodiment. Besides, it is also possible to use any desired opticalwaveguides as the input waveguides and output waveguides byinterconnecting any desired optical waveguides. Furthermore, it is alsopossible to combine three or more interferometer optical switches withthe same structure, or to combine a plurality of interferometer opticalswitches with different structures.

As described above, using the interferometer optical switch of thepresent embodiment enables the switching operation over the broadwavelength band. In addition, since the interferometer optical switch ofthe present embodiment can carry out switching operation over the broadwavelength band, it has a great tolerance for the power coupling ratioerror of the optical multi/demultiplexing devices or for the path lengthdifference error of the optical delay lines. Accordingly, the presentembodiment implements an interferometer optical switch that can maintaina high extinction ratio even if there is fabrication error.

(First Variation of Fourth Embodiment)

FIG. 20 shows a configuration of the interferometer optical switch of afirst variation of the fourth embodiment in accordance with the presentinvention. The circuit of the variation is an optical switch with amultiple-stage interferometer configuration using two interferometeroptical switches described in the second embodiment as shown in FIG. 13.With such a multiple-stage interferometer configuration, the switch canachieve an extinction ratio higher than that of the single basiccomponent because it can prevent the leakage light with the plurality ofbasic components 170 and 171 in the OFF state.

The multiple-stage interferometer optical switch of the present examplehas two basic components of FIG. 13 arranged in line symmetry withrespect to the center of the circuit. Then, the interferometer opticalswitch 170 in the first stage has its first output (corresponding to 104of FIG. 13) connected to a first input (corresponding to 102 of FIG. 13)of the interferometer optical switch 171 in the second stage, and hasits second output (corresponding to 103 of FIG. 13) used as the outputwaveguide 103. The interferometer optical switch 171 in the second stagehas its second input (corresponding to 101 of FIG. 13) used as the inputwaveguide 101. The input waveguide 101 and the output waveguide 103intersect with each other on the way, thereby forming a cross waveguide155. In addition, the interferometer optical switch 170 of the firststage has its second input (corresponding to 101 of FIG. 13) used as theinput waveguide 102, and the interferometer optical switch 171 of thesecond stage has its second output (corresponding to 103 of FIG. 13)used as the output waveguide 104. It is obvious that the two basiccomponents 170 and 171 can be disposed in the same direction as in theforegoing fourth embodiment or can be disposed in the oppositedirection. As for the circuit layout, it is not limited: the two basiccomponents 170 and 171 can be disposed in the horizontal direction asshown in FIG. 20, or in the vertical direction.

The interferometer optical switches 170 and 171 constituting themultiple-stage interferometer of this example use the same designvalues. The power coupling ratios of the two directional couplers 151and 152 and 153 and 154, and the path length differences of the minuteoptical delay lines 132 and 133 constituting the phase generatingcouplers were obtained by using the multiple regression analysis in sucha manner that the power coupling ratios of the phase generating couplers111 and 112 (see FIG. 13) became about 0.5 at the center wavelengthλc=1.55 μm of the wavelength region and that the phase differencebetween the output light satisfied the foregoing expression (9). As aresult, the power coupling ratios of the directional couplers 151 and152 constituting the first phase generating coupler 111 were set atr₁=0.3 and r₂=0.1, respectively, and the path length difference of theminute optical delay line 132 was set at ΔL₁=0.19λc (≈0.29 μm). Here,the path length difference represents the relative optical path lengthof the upper optical waveguide with respect to the lower opticalwaveguide. Likewise, the power coupling ratios of the directionalcouplers 153 and 154 constituting the second phase generating coupler112 were set at r₁=0.1 and r₂=0.3, respectively, and the path lengthdifference of the minute optical delay line 133 was set at ΔL₂=0.19λc(≈0.29 μm). In addition, the path length difference of the Mach-Zehnderinterferometer was set at ΔL=0.16λc (≈0.25 μm), and the spacing betweenthe two optical waveguides interconnecting the opticalmulti/demultiplexing devices 111 and 112 was made 100 μm. As the phaseshifters 141, a thin film heater was used and its width was set at 40μm, and length at 4 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was1.5%, the core cross section of the optical waveguides was 4.5×4.5 μm²,and the width and the depth of the adiabatic grooves 168 were 70 μm and50 μm, respectively.

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module. Then, the switching characteristics of the interferometeroptical switch module were evaluated.

When the phase shifters (thin film heaters) 141 are in the OFF state,the switch is in the bar state. Thus, the optical signal input via theinput waveguide 102 is output from the output waveguide 103, but notfrom the output waveguide 104. In this case, since the two basiccomponents 170 and 171 prevent leakage light from coming out of theoutput waveguide 104, the present embodiment can achieve a higherextinction ratio. By supplying power to the thin film heaters 141 of theinterferometer optical switches 170 and 171, and varying the opticalpath length by an amount corresponding to half the wavelength of theoptical signal (0.5λc·k: k is an integer other than zero) by thethermooptic effect, the path length difference becameΔL−δΔL=0.16λc−0.50λc=−0.34λc. In this case, the phase shifters (thinfilm heaters) 141 were in the ON state, and the switch was in the crossstate, and hence the optical signal input via the input waveguide 102was output from the output waveguide 104. In addition, although thepresent example employs the waveguides 101 and 102 as the inputwaveguides, it can achieve the same advantages by using the waveguides103 and 104 as the input waveguides, and 101 and 102 as the outputwaveguides. Besides, since the optical switch of the present example hasthe adiabatic groove structure, it can suppress the power consumption ofthe phase shifters required for the switching to 1/10 that of theconventional switch.

Next, FIG. 21 illustrates the wavelength characteristics of the measuredtransmittance of the interferometer optical switch of the presentexample. The wavelength dependence of the transmittance of theconventional Mach-Zehnder interferometer optical switch as shown in FIG.37 is also illustrated for comparison.

When the phase shifters 141 are in the OFF state, the interferometeroptical switch of the present example can achieve a high extinctionratio equal to or greater than 60 dB over a broad wavelength band of1.45-1.65 μm, and equal to or greater than 80 dB over a broad wavelengthband of 1.45-1.63 μm because of the multiple-stage interferometerconfiguration. When the phase shifters are brought into the ON state,the interferometer optical switch of the present example achieves a goodinsertion loss over a broad wavelength band.

As described above, using the plurality of interferometer opticalswitches in accordance with the present invention enables them tofunction as a single interferometer optical switch. Although the presentexample constructs the two-stage interferometer configuration bycombining the two identical interferometer optical switches, it isobvious that two interferometer optical switches can use differentdesign values. In addition, it is also possible to assume aconfiguration other than the two-stage interferometer configurationdescribed in the present example such as combining the interferometeroptical switch of the first embodiment and the interferometer opticalswitch of the second embodiment. Besides, the method of interconnectinga plurality of interferometer optical switches is not limited to that ofthe present example, but any desired optical waveguides can beinterconnected, and any desired optical waveguides can be used as theinput waveguides and output waveguides. Furthermore, it is also possibleto combine three or more interferometer optical switches.

To increase the tolerance for the fabrication error, the present examplewere designed such that the power coupling ratios of the directionalcouplers 151-154 and the path length differences of the minute opticaldelay lines 132 and 133 constituting the two phase generating couplers111 and 112 (see FIG. 13) became equal, respectively. Then, the phasegenerating couplers 111 and 112 are configured in line symmetry withrespect to the center, which means that r₁=r₄, r₂=r₃ and ΔL₁=ΔL₂. Therewill only be two types of a coupling ratio design for the directionalcouplers 151-154. Accordingly, the switch described in the presentembodiment can realize the designed switching characteristics simply byfabricating two types of the coupling ratio. In contrast, if all fourdirectional couplers were designed with different coupling ratios, asdescribed in the second embodiment, it will be necessary to fabricateall four types of the coupling ratios to the designed values. Therefore,the switch design in the present embodiment has larger fabricationtolerance. On the other hand, the switch design in the second embodimentoffers higher degree of approximation because there is more flexibilityin the design variables. The preferred circuit design can be selecteddepending on the use of the interferometer optical switches.

In addition, comparing the two configurations of the multiple-stageinterferometers, although the present example is greater than theforegoing fourth embodiment in the circuit size, it has the advantage ofbeing able to reduce the types of the design values. More specifically,although the fourth embodiment uses three types of directional couplerswith different power coupling ratios, the present example uses only twotypes of the power coupling ratios, thereby facilitating thefabrication. Furthermore, the present example constructs themultiple-stage interferometer switch by placing two basic switches inline symmetry, where the basic switches each has line symmetricconfiguration. Thus, the present example has a configuration with a veryhigh symmetry, which facilitates the insertion of a half-wave plate orthe like.

Although the two interferometer optical switches 170 and 171 have thesame design values in the present example, they can have differentdesign values. For example, since the present example sets the maximumextinction wavelength of the two interferometer optical switches at 1.55μm, it can implement a maximum extinction ratio with a very highabsolute value equal to or greater than 140 dB in the maximum extinctionwavelength range of 1.52-1.57 μm around the center wavelength of 1.55μm. On the other hand, the maximum extinction wavelength of the twoswitches can be set at different values. For example, the maximumextinction wavelength of the interferometer optical switch 170 can beset at about 1.5 μm, and the maximum extinction wavelength of theinterferometer optical switch 171 can be set at about 1.6 μm. Althoughthere will be a little reduction of the maximum extinction, thewavelength range with maximum extinction can be extended. The embodimentdescribed here is only an example of the possible implementation of thepresent invention, and the basic components constituting themultiple-stage interferometer can be designed to have any desiredcharacteristics.

FIFTH EMBODIMENT

FIG. 22 shows a configuration of the interferometer optical switch of afifth embodiment in accordance with the present invention. The circuitof the interferometer optical switch of the present embodiment includesa phase generating coupler 111; a directional coupler 153; an opticaldelay line 131 between the optical multi/demultiplexing device 111 anddirectional coupler 153; a phase shifter 141 formed in the optical delayline 131; input waveguides 101 and 102; and output waveguides 103 and104.

As for the interferometer optical switches of the foregoing first tofourth embodiments, such cases as satisfying the foregoing expression(7) are described so that the output intensity is switched between 0 and1 in particular. However, the optical switch in accordance with thepresent invention can be configured in such a manner that the outputintensity can take a different value between 0 and 1 by setting the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the phase differences at a value differentfrom m·π (m is an integer) so that the sum becomes wavelengthinsensitive. This makes it possible to implement an output intensityvariable optical switch (broad band variable optical attenuator) thatcan be used over a broad band.

To place the output transmittance at 0 dB, −10 dB, −20 dB and −30 dB,for example, the phase difference values are set such that the outputintensity of the optical signal output from the output waveguide 104 isPc=1.0, 0.1, 0.01 and 0.001. Since the output intensity of the opticalswitch is represented by the foregoing expression (6), setting the totalphase difference {φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} caused by the opticalmulti/demultiplexing device 111 and optical delay line 131 at −1.00,−0.60, −0.53, −0.51 and −0.50 can implement the broad band lightintensity variable optical switch with the output transmittance of 0 dB,−10 dB, −20 dB and −30 dB.

In the interferometer optical switch of the present embodiment as shownin FIG. 22, the power coupling ratios of the directional couplers 151and 152 constituting the phase generating coupler 111 were set at r₁=0.3and r₂=0.7, and the optical path length of the minute optical delay line132 is set at ΔL₁=0.30λc (≈0.47 μm). In addition, the path lengthdifference of the Mach-Zehnder interferometer 131 was set at ΔL=0.34·λc(≈0.53 μm), and the power coupling ratio of the directional coupler 153was set at r₃=0.5. Here, the path length difference represents therelative optical path length of the upper optical waveguide with respectto the lower optical waveguide. The spacing between the two opticalwaveguides interconnecting the optical multi/demultiplexing device 111and the directional coupler 153 was made 200 μm. As the phase shifter141, a thin film heater was used and its width was set at 40 μm, andlength at 4 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was1.5% and the core cross section of the optical waveguides was 4.5×4.5μm².

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heater 141, thereby forming a two-input, two-output opticalswitch module. Then, the switching characteristics of the interferometeroptical switch module were evaluated.

When the phase shifter (thin film heater) 141 is in the OFF state, theswitch is in the bar state. When light is launched into the inputwaveguide, in this state, most of the light is launched from the outputwaveguide 103, and light launched from the output waveguide 104 isblocked. FIG. 23 illustrates the wavelength dependence of thetransmittance under the assumption that the wavelength dependence of thetransmittance in this state corresponds to the case of the maximumextinction. Here, supplying power to the thin film heater 141 andvarying the power to set the optical path length difference atΔL=0.35λc, 0.37λc, 0.44λc and 0.84λc by the thermooptic effect, thetotal phase difference {φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} caused by the opticalmulti/demultiplexing device 111 and optical delay line 131 became −0.51,−0.53, −0.60 and −1.00, and the transmittance in the respective statebecame −30 dB, −20 dB, −10 dB and 0 dB (see FIG. 23). In addition, thewavelength dependence of the transmittance was wavelength insensitiveover a broad wavelength band as illustrated in FIG. 23. Comparing withthe wavelength characteristics of the conventional variable opticalattenuator as illustrated in FIG. 41B, the advantages of the presentinvention are quite obvious. The conventional switch can attenuate thelight at only a particular wavelength. In contrast, the presentinvention can attenuate the light collectively over a broad wavelengthband. Thus, we confirmed the operation of the interferometer opticalswitch as a wide range intensity variable optical switch (variableoptical attenuator).

Although the ideal case was described here in which the first and secondoptical multi/demultiplexing devices 111 and 153 each have the powercoupling ratio of 0.5, the wavelength dependence or fabrication errorcan take place in practice. To achieve the precise flatness, the phasedifference should be set considering the power coupling ratios of thefirst and second optical multi/demultiplexing devices 111 and 153.Assume that the first and second optical multi/demultiplexing devices111 and 153 have the power coupling ratios R1(λ) and R2(λ),respectively. Then, by setting the sum of the phase differences in sucha manner that the output intensity Pc(λ) becomes constant for thewavelength, the output intensity can be made wavelength insensitive.More specifically, the sum of the phase differences is preferably set asfollows.φ₁(λ)+φ_(ΔL)(λ)+φ₂(λ)=arcos{{Pc(λ)−R1(λ){1−R2(λ)}−R2(λ){1−R1(λ)}}·[4R1(λ){1−R2(λ)}R2(λ){1−R1(λ)}]^(−1/2)}/2π

As clearly seen from the characteristics of FIG. 23, by only setting thesum of the phase differences at a constant value, the transmittance canbe kept constant over a sufficiently broad band, thereby being able toimplement good characteristics.

Although the optical signal is input to the input waveguide 101, and theoptical signal is output from the output waveguide 104 in the foregoingexample, this is not essential. For example, the optical signal can beoutput from the output waveguide 103, or the optical signal can be inputto the input waveguide 102. Alternatively, the optical signal can beinput to the output waveguides 103 and 104, and output from the inputwaveguides 101 and 102. In addition, although not shown in FIG. 22, aphase shifter can also be formed on the lower side optical waveguide(second optical waveguide) of the two delay lines constituting theoptical delay line 131 to vary the optical path length difference, whichmakes it possible to set the optical attenuation at the desired value.It is obvious that the intensity variable optical switch described inthe present embodiment can implement the features described in the otherembodiments. For example, as described in the second embodiment, thefirst and second optical multi/demultiplexing devices 111 and 112 ofFIG. 13 can be used as the phase generating couplers; as described inthe third embodiment, the adiabatic groove structure of FIG. 15 can beformed; and as described in the fourth embodiment, the multiple-stageinterferometer configuration of FIG. 18 can be used to make the totaloutput intensity constant with respect to the wavelength by adjustingeach basic components with different conditions.

As described above, the interferometer optical switch described in thepresent embodiment uses a novel operation principle to implement aswitch that is operatonal over a wide wavelength region. It wasconfirmed that the switch operates as a variable optical attenuator withuniform transmittance throughout the whole wavelength region. Inaddition, the switch is operational with only one phase shifter.

SIXTH EMBODIMENT

FIG. 24 shows a configuration of the interferometer optical switch of asixth embodiment in accordance with the present invention. The circuitof the interferometer optical switch of the present embodiment includesa pair of optical multi/demultiplexing devices (phase generatingcouplers) 111 and 112, the phase differences of the outputs of whichhave the wavelength dependence; an optical delay line 131 between theoptical multi/demultiplexing devices 111 and 112; phase shifters 141formed in the optical delay line 131; input waveguides 101 and 102; andoutput waveguides 103 and 104. Setting the phase differences of theoutputs of the optical multi/demultiplexing devices 111 and 112appropriately makes it possible to implement an optical switch with goodswitching characteristics over a broad wavelength band.

Although a variety of means are conceived as a method of implementingthe optical multi/demultiplexing devices, the phase differences of theoutputs of which have the wavelength dependence, the present embodimentconfigures the optical multi/demultiplexing devices 111 and 112 with N+1optical couplers, and N optical delay lines sandwiched between theadjacent optical couplers, where N is a natural number. FIG. 24 shows acase where N=2.

FIG. 25 shows a configuration of the phase generating coupler (opticalmulti/demultiplexing device) used in the sixth embodiment in accordancewith the present invention. The optical multi/demultiplexing device ofFIG. 25 includes three optical couplers 123, 124 and 125, and twooptical delay lines 132 and 133 between the adjacent optical couplers.The optical delay line 132 is composed of two optical waveguides: afirst optical waveguide 156, and a second optical waveguide 158, theoptical path lengths of which are given by l_(1a) and l_(2a). Theoptical path length difference is δ₁=l_(1a)−l_(2a). Likewise, theoptical delay line 133 is composed of two optical waveguides: a firstoptical waveguide 157, and a second optical waveguide 159, the opticalpath lengths of which are given by l_(1b) and l_(2b). The optical pathlength difference is δl₂=l_(1b)−l_(2b).

The first to fifth embodiments described so far employed the opticalmulti/demultiplexing device composed of N+1 optical couplers and Noptical delay lines as a means for implementing the phase generatingcoupler. This is because this device can be designed to function as aphase generating coupler with desired coupling ratio and output phasedifference with no theoretical loss. It is obvious that other devicescan be implemented as a phase generating coupler to create awavelength-dependent phase required for producing the switches describedin the present invention. For example, a combination of optical couplersand an optical delay line can be used to configure the opticalmulti/demultiplexing device. It may be an FIR (Finite Impulse Response)filter typified by a transversal-form filter, or an IIR (InfiniteImpulse Response) filter typified by a ring-form filter.

Next, a design example will be described in which the interferometeroptical switch is operated as an asymmetric optical switch. Theasymmetric switch is achieved when the total phase obtained by summingup the phase difference 2πφ_(ΔL)(λ) caused by the optical path lengthdifference of the optical delay line of the Mach-Zehnder interferometer,and the phase differences 2πφ₁(λ) and 2πφ₂(λ) produced by the phasegenerating couplers 111 and 112, the phase differences of the outputs ofwhich have wavelength dependence, is equal to m·π (m is an integer), andwhen m is an odd number. A conventional asymmetric Mach-Zehnderinterferometer optical switch has wavelength dependence that comes fromthe optical delay line. Accordingly, it cannot set the phase at m·π (m:odd number) except for a particular wavelength, and hence the usablewavelength band is limited. On the other hand, the interferometeroptical switch in accordance with the present invention can set thephase at a constant value m·π (m: odd number) regardless of thewavelength by using the optical multi/demultiplexing devices (phasegenerating couplers) 111 and 112, the phase differences of the outputsof which have wavelength dependence. In addition, since the switch isasymmetric, the cross port has a high extinction ratio even if the powercoupling ratios of the first and second optical multi/demultiplexingdevices 111 and 112 deviate from the ideal value 0.5. There will be aninsertion loss in ON state when the power coupling ratio deviates fromthe ideal value, but the loss is negligible compared with thedeterioration of the extinction ratio of the symmetric switch when thepower coupling ratio deviates from the ideal value.

In the first embodiment described with reference to FIG. 1, theforegoing expression (6) is obtained under the assumption that the firstand second optical multi/demultiplexing devices 111 and 112 have thepower coupling ratio of a constant value 0.5 throughout the wavelengthband. In practice, however, it is not easy to set the power couplingratios of the optical multi/demultiplexing devices at a constant valueof 0.5 throughout the wavelength band. In particular, as the wavelengthband becomes broader, it becomes more difficult to maintain the powercoupling ratio at a constant value. If the first and second opticalmulti/demultiplexing devices 111 and 112 have the same power couplingratio R(λ), the light intensity Pc that is output from the outputwaveguide 104 by the input to the waveguide 101 is given by thefollowing expression.P _(C)=2R(λ)·[1−R(λ)]·[1+cos{2π{φ_(ΔL)(λ)+Φ(λ)}}]  (10).

Where Φ(λ) is a phase produced by the phase differences of the outputsof the first and second optical multi/demultiplexing devices 111 and112, and Φ(λ)≡φ₁(λ)+φ₂(λ). It is seen from the foregoing expression (10)that when 2π{φ_(ΔL)(λ)+φ₁(λ)+φ₂(λ)} is equal to m·π (m is an integer)and m is an odd number, high extinction ratio can be maintainedregardless of the power coupling ratio R(λ) of the first and secondoptical multi/demultiplexing devices 111 and 112. Thus, it is easy tomaintain a high extinction ratio over a broad wavelength band.

In summary, a conventional asymmetric Mach-Zehnder interferometeroptical switch could not operate over a wide wavelength region becausethere will be wavelength dependence when the optical path lengthdifference of the optical delay line is set at a finite value. Incontrast, the optical path length difference of the optical delay linecan be set at arbitrary value by providing the optical delay line with awavelength-dependenent phase. This phase is generated by the phasedifference of the output ports of the optical multi/demultiplexingdevice. Thus, implementing this principle produces an asymmetricMach-Zehnder interferometer optical switch with a high extinction ratioover a wide wavelength region and with large fabrication tolerance.

Next, a concrete design example of the phase generating coupler will bedescribed. The present embodiment uses as a means for implementing thephase generating coupler an optical multi/demultiplexing deviceincluding N+1 optical couplers and N minute optical delay linessandwiched between the adjacent optical couplers. Then, under theconstraints that the N+1 optical couplers constituting the first andsecond optical multi/demultiplexing devices 111 and 112 all have thesame power coupling ratio, and its value was made as small as possible,the design parameters were optimized so that the power coupling ratiosof the phase generating couplers became about 0.5 throughout thewavelength band used, and that the output phase difference φ(λ) wasequal to the required phase Ψ(λ). The optimized design parameters are asfollows: the power coupling ratios of the optical couplers 151, 152,153, 251, 252 and 253 constituting the phase generating couplers; theoptical path length differences of the minute optical delay lines 132,133, 232 and 233; and the optical path length difference ΔL of theoptical delay line 131 of the Mach-Zehnder interferometer. Although theoptical path length difference of a conventional Mach-Zehnderinterferometer optical switch is set at 0·λc or 0.5·λc, the presentinvention optimizes the output phase differences of the phase generatingcouplers including the optical path length difference ΔL to approximatethe phase, which is one of the characteristics of the present invention.

In the present embodiment, the wavelength range is set at 1.25-1.65 μm,and considering that the switch is mainly used at 1.3 μm and 1.55 μm,the optimization is made such that the degree of approximation becomesmaximum at 1.3 μm and 1.55 μm in particular. As the optical couplers151, 152, 153, 251, 252 and 253, directional couplers each consisting oftwo optical waveguides placed side by side in close proximity are used.As a result of the optimization, N was given by N=2; the power couplingratio of the directional couplers 151, 152, 153, 251, 252 and 253 becamer=0.1; the optical path length difference of the minute optical delaylines 132 and 232 became ΔL₁=0.09·λc (≈0.13 μm); and the optical pathlength difference of the minute optical delay lines 133 and 233 becameΔL₂=0.05·λc (≈0.07 μm). In addition, the optical path length differenceof the Mach-Zehnder interferometer was made ΔL=0.31·λc(≈0.45 μm), m wasgiven by m=−1, and the spacing between the two optical waveguides acrossthe optical multi/demultiplexing devices 111 and 112 was made 500 μm. Asthe phase shifters 141, a thin film heater was used whose width was 80μm, and length was 3 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75%, and the core cross section of the optical waveguides was 6×6 μm².

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module.

When the phase shifters (thin film heaters) 141 are in the OFF state,the switch is in the bar state, and hence the optical signal is notoutput from the cross port. By supplying power to the thin film heaters141, the optical path length difference is changed toΔL+δΔL=0.31λc−0.50λc=−0.19λc, thereby changing the switch to the ONstate. In this state, the switch is turned into the cross state, and theoptical signal is output from the cross port.

Next, FIG. 26 illustrates the wavelength characteristics of thetransmittance measured for the interferometer optical switch of thepresent embodiment. When the phase shifters 141 are in the OFF state,the optical switch of the present embodiment can achieve a highextinction ratio equal to or greater than 30 dB over a broad wavelengthband of 1.25-1.6 μm. In particular, since the present embodimentoptimizes the design values of the phase generating couplers 111 and 112in such a manner that the degree of approximation becomes high at 1.3 μmand 1.55 μm, the extinction ratio is higher at 1.3 μm and 1.55 μm, whichis equal to or greater than 50 dB.

As described above, the present invention provides an asymmetricMach-Zehnder interferometer optical switch that functions as a gateswitch capable of maintaining a high extinction ratio over a wavelengthregion. The present invention, however, is not limited to the gateswitch, but is applicable to broaden the band of any desiredinterferometer optical switches such as broad band tap switches. Inaddition, although the phase differences of the phase generatingcouplers and the optical path length difference ΔL of the optical delayline of the Mach-Zehnder interferometer are set at2π{φ_(ΔL)(λ)+φ₁(λ)+φ₂(λ)}=(2m′+1)·π (m′ is an integer) in the initialOFF state in the foregoing description, this is not essential. Forexample, setting them at 2π{φ_(ΔL)(λ)+φ₁(λ)+φ₂(λ)}=2m′·π (m′ is aninteger) in the OFF state can implement a symmetric Mach-Zehnderinterferometer which is capable of switching over a broad wavelengthband, and which can operate as a good bifurcation switch.

For the optimization of the design variables, the present embodimentimpose certain constraints where all of the N+1 optical couplers (123,124 and 125) have the same power coupling ratio, and its value is madeas small as possible. Although such constraints are not essential,setting all the power coupling ratios of the optical couplersconstituting the phase generating couplers at a constant value offers anadvantage of being able to facilitate the fabrication of the opticalcouplers. In addition, although the power coupling ratios can be set atany desired values from zero to one, the power coupling ratios areoptimized at a small value such as 0.1. This is because the reduction ofthe power coupling ratios offers such advantages as downsizing thedirectional couplers, increasing the fabrication tolerance, anddecreasing the polarization dependence. Since the phase generatingcouplers (FIG. 25) of the present embodiment each have a larger numberof optical couplers (123, 124 and 125) and optical delay lines (132 and133) than the case where N=1 (FIG. 4), the circuit size is slightlyincreased. However, since the directional couplers (123, 124 and 125)are compact, an increase in the circuit size is negligible.

In addition, the present embodiment employs the phase generatingcouplers composed of N+1 optical couplers, and N optical delay linessandwiched between the adjacent optical couplers, where N is set at N=2.An increase of N, which will enable an increase of the number ofparameters that can be set, can raise the degree of approximation of thephase generating couplers. For example, comparing FIG. 26 (the presentembodiment) with FIG. 17 (third embodiment), since the presentembodiment is better in the degree of approximation, it has a broaderwavelength range in which the extinction ratio is equal to or greaterthan 30 dB. More specifically, the third embodiment, which sets N atN=1, has three design variables. In contrast, the present embodiment,which sets N at N=2, has five design variables, and hence has higherdesign flexibility. Accordingly, the present embodiment can raise thedegree of approximation. There will still be sufficient designflexibility even when N+1=3 optical couplers are set at the same value.As a result, the design values are obtained which enable all the opticalcouplers to have the power coupling ratio of 0.1, and hence canimplement the interferometer optical switch having large resistance tothe fabrication error, and small polarization dependence.

Furthermore, the present embodiment employs two phase generatingcouplers 111 and 112, and in their optical delay lines 132, 133, 233 and232, the optical delay line having greater sum of the optical pathlengths is placed at one side (upper side of FIG. 24) unevenly on thecircuit. More specifically, in FIG. 24, the optical path lengths of thefirst optical waveguide constituting the N=2 optical delay lines 132 and133 of the first optical multi/demultiplexing device 111 have the sum ofΣδl_(1,1)=l_(11a)+l_(11b)=678.26+551.79=1230.05; the optical pathlengths of the second optical waveguide have the sum ofΣl_(2,1)=l_(21a)+l_(21b)=678.13+551.72=1229.85; the optical path lengthsof the first optical waveguide constituting the N=2 optical delay line233 and 232 of the second optical multi/demultiplexing device 112 havethe sum of Σl_(1,2)=l_(12a)+l_(12b)=551.79+678.26=1230.05; and theoptical path lengths of the second optical waveguide have the sum ofΣl_(2,2)=l_(22a)+l_(22b)=551.72+678.13=1229.85. Thus, they satisfy therelations Σl_(1,1)>Σl_(2,1) and Σl_(1,2)>Σl_(2,2), which means that thefirst optical waveguide has the longer sum of the optical delay lines ofthe phase generating couplers 111 and 112. Accordingly, it is seen thatthe longer waveguide are disposed unevenly at the upper side of FIG. 24.In this way, the phase generating couplers 111 and 112 can produce thephase effectively phase. In particular, the configuration of theinterferometer optical switch of the present embodiment is a specialexample, which uses as the first and second optical multi/demultiplexingdevices the phase generating couplers with the same design values, anddisposes them in such a manner that they are mirror symmetry with eachother with respect to the center of the optical delay line 131 of theMach-Zehnder interferometer. In this case, the phase differences are setin such a manner that they satisfy the relationship φ₁(λ)=φ₂(λ)=Ψ(λ)/2.The first embodiment employs a phase difference φ₁(λ) of only one phasegenerating coupler. In contrast, it is enough for the present embodimentto produce half of the required phase difference per phase generatingcoupler. It is obvious that it is not essential for the longer waveguidein the optical delay lines of the first and second phase generatingcouplers to be disposed at one side, but disposed at opposite sides.Incidentally, in the individual embodiments in accordance with thepresent invention, the path length difference of the optical delay linerefers to the relative path length difference of the first waveguidewith respect to the second waveguide. Accordingly, when the secondwaveguide is longer than the first waveguide, the path length differencebecomes negative. When N is equal to or greater than two, and N opticaldelay lines have different signs, the uneven disposition can be definedin the same manner as described above. For example, if the secondwaveguide of the minute optical delay lines 132 and 232 is longer, andhence the path length difference has a negative sign, the sum of theoptical path length differences of the first optical waveguide andsecond optical waveguide constituting the optical delay lines 132 and133 of the first optical multi/demultiplexing device 111, and the sum ofthe optical path lengths of the first optical waveguide and secondoptical waveguide constituting the optical delay line 233 and 232 of thesecond optical multi/demultiplexing device 112 areΣl_(1,1)=l_(11a)+l_(11b)=678.13+551.79=1229.92,Σl_(2,1)=l_(21a)+l_(21b)=678.26+551.72=1229.98,Σl_(1,2)=l_(12a)+l_(12b)=551.79+678.13, andΣl_(2,2)=l_(22a)+l_(22b)=551.72+678.26, respectively. In this case, theysatisfy Σl_(2,1)>Σl_(1,1) and Σl_(2,2)>Σl_(1,2), which means that theoptical delay lines of the first and second multi/demultiplexing device111 and 112 are both disposed in the second waveguide side.

SEVENTH EMBODIMENT

FIG. 27 shows a configuration of the interferometer optical switch of aseventh embodiment in accordance with the present invention. It ispossible to make the interferometer optical switch of the presentembodiment function as a 1×2 switch. In addition, it can carry out theswitching operation regardless of the wavelength by using phasegenerating couplers, the phase differences of the outputs of which havewavelength dependence, as the optical multi/demultiplexing devices ofthe interferometer optical switch, the basic component of the presentcircuit.

The circuit of the optical switch is configured by connecting twointerferometer optical switches 170 and 171 in cascade. Morespecifically, the two, first and second, interferometer optical switches170 and 171 with the same design values are used, and the upper port atthe output side of the first interferometer optical switch 170 isconnected to the lower port at the input side of the secondinterferometer optical switch 171. In addition, the upper port at theinput side of the first interferometer optical switch 170 is used as theinput waveguide 101; the upper port at the output side of the secondinterferometer optical switch 171 is used as the output waveguide 103(first output port); and the lower port at the output side of the firstinterferometer optical switch 170 is used as the output waveguide 104(second output port).

Since the present embodiment employs two interferometer optical switches170 and 171 with the same design values, only the first interferometeroptical switch 170 will be described in detail. Although a variety ofmeans are conceivable as a method of implementing the opticalmulti/demultiplexing device, the phase difference of the output of whichhas wavelength dependence, the optical multi/demultiplexing device canbe implemented by interconnecting optical couplers with an optical delayline. The present embodiment configures each of the opticalmulti/demultiplexing devices (phase generating coupler) 111 and 112 withN+1 (=2) optical couplers 151 and 152 or 153 and 154; and N (=1) opticaldelay line 132 or 133 between the adjacent optical couplers. The opticalmulti/demultiplexing device 111 includes the optical couplers(directional couplers 151 and 152), and the optical delay line 132between the adjacent optical couplers. The optical delay line 132 iscomposed of two optical waveguides: a first optical waveguide and asecond optical waveguide, and their optical path length difference isΔL₁=l₁₁−l₂₁, where l₁₁ and l₂₁ are their optical path lengths. Likewise,the optical multi/demultiplexing device 112 includes the opticalcouplers (directional couplers 153 and 154), and the optical delay line133 between the adjacent optical couplers. The optical delay line 133 iscomposed of two optical waveguides: a first optical waveguide and asecond optical waveguide, and their optical path length difference isΔL₂=l₁₂−l₂₂, where l₁₂ and l₂₂ are their optical path lengths.

Next, a design example of a concrete phase generating coupler will bedescribed. The present embodiment uses as a means for implementing thephase generating coupler an optical multi/demultiplexing device composedof N+1 optical couplers, and N minute optical delay lines sandwichedbetween the adjacent optical couplers. Then constraints are imposed thatthe power coupling ratios of each N+1=2 (four in total) optical couplersconstituting the first and second optical multi/demultiplexing devices111 and 112 are the same (r₁=r₂=r₃=r₄=r). As the optical couplers,directional couplers are used each of which is composed of two opticalwaveguides placed side by side in close proximity. In the presentembodiment, the wavelength range is set at 1.45-1.65 μm, and the designparameters are optimized in such a manner that the power coupling ratioof the phase generating coupler becomes about 0.5 throughout thewavelength band used, and the phase difference Φ(λ)=φ₁(λ)+φ₂(λ) of theoutput agrees with the appropriate phase Ψ(λ). Here, the appropriatephase refers to a phase required for implementing an asymmetricMach-Zehnder interferometer optical switch capable of operation over awide wavelength range. The phase is given by substituting 2m′+1 (m′ isan integer) for m (m is an integer) of the foregoing expression (7). Theoptimized design parameters include the power coupling ratios of theoptical couplers constituting the phase generating couplers; the opticalpath lengths of the minute optical delay lines; and the optical pathlength difference ΔL of the optical delay line of the Mach-Zehnderinterferometer. A conventional Mach-Zehnder interferometer opticalswitch has an optical path length difference of ΔL, which is set at 0·λcor 0.5·λc. In contrast, the present invention performs optimization ofthe phase generating coupler design variables including the optical pathlength difference ΔL so that the phase difference of the light launchedfrom the phase generating coupler is equal to the appropriate phase.

For the optimization, the present embodiment imposes the constraintsthat the N+1 optical couplers have the same power coupling ratio.Although such constraints are not essential, setting all the powercoupling ratios of the optical couplers constituting the phasegenerating couplers at the same value offers the advantage of being ableto facilitate the fabrication of the optical couplers. Comparing withthe second embodiment, since the second embodiment does not impose anyconstraints on the power coupling ratios of the optical couplersconstituting the phase generating couplers, and increases theflexibility of the design values, the second embodiment is superior tothe present embodiment in the degree of approximation. In contrast, thepresent embodiment is designed considering the ease of fabrication. Likethis way, the phase generating couplers are designed according to theapplication of the circuit.

Furthermore, the present embodiment employs two phase generatingcouplers, and their optical delay lines are placed on the circuit insuch a fashion that the optical delay line with a greater sum of theoptical path lengths is unevenly disposed at one side (upper side ofFIG. 27), that is, in such a manner that Σl_(1,1)>Σl_(2,1) andΣl_(1,2)>Σl_(2,2) are satisfied.

As a result of numerical calculations, N was set at N=1; the powercoupling ratio of the directional couplers 151, 152, 153 and 154 was setat r=0.2; and the optical path length differences of the minute opticaldelay lines 132 and 133 were set at ΔL₁=ΔL₂=0.15·λc (≈0.23 μm). Inaddition, the optical path length difference of the Mach-Zehnderinterferometer was set at ΔL=0.28·λc (≈0.43 μm); m′ was set at m′=−1;and the spacing between the two optical waveguides that connects opticalmulti/demultiplexing devices 111 and 112 was made 100 μm. As the phaseshifters, a thin film heater was used, whose width was 30 μm, and lengthwas 2 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75% and the core cross section of the optical waveguides was 6×6 μm².

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module.

Next, the switching operation will be described in the case where alight is input via the input waveguide 101 of the optical switch modulefabricated, and the output port is switched from the output waveguide103 (first output port) to 104 (second output port). In the initialstate, the thin film heater (144) is activated to change the opticalpath length difference of the interferometer optical switch 171 toΔL+δΔL=0.28λc−0.50λc=−0.22λc. In this state, the interferometer opticalswitch 170 of the first stage is in the bar state, and theinterferometer optical switch 171 of the second stage is in the crossstate. Accordingly, the light input via the input waveguide 101 iscompletely transmitted via the through port of the interferometeroptical switch 170, and is output from the output waveguide 103 (firstoutput port) of the interferometer optical switch 171. On the otherhand, the light is not output from the output waveguide 104 (secondoutput port) of the interferometer optical switch 170. Next, the thinfilm heater (144) is turned off again, and the power is supplied to thethin film heater (142) to change the optical path length difference ofthe interferometer optical switch 170 to ΔL+δΔL=0.28λc−0.50λc=−0.22λc.In this state, the interferometer optical switch 170 of the first stageis placed in the cross state, and the interferometer optical switch 171of the second stage is placed in the bar state. Accordingly, the lightinput via the input waveguide 101 is output from the cross port of theinterferometer optical switch 170, that is, from the output waveguide104 (second output port). On the other hand, since the light is cut offby the through port of the interferometer optical switch 170 and thecross port of the interferometer optical switch 171, the light is notoutput from the output waveguide 103 (first output port). In this way,the interferometer optical switch of the present embodiment can beoperated as a bifurcation switch with a constant power consumption of0.5 W.

FIG. 28A illustrates the wavelength characteristics of the transmittancein the initial state (OFF state) output from the output waveguide 103(first output port) of the interferometer optical switch of the presentembodiment; and FIG. 28B illustrates the wavelength characteristics ofthe transmittance in a post-switching state (ON state) output from theother output waveguide 104 (second output port). In either case, a highextinction ratio equal to or greater than 30 dB is obtained over a broadwavelength band of 1.45-1.65 μm. Thus, a 1×2 switch is implemented whichhas a high extinction ratio over a broad wavelength band, and constantpower consumption.

In the present embodiment, two interferometer optical switches equippedwith phase generating couplers were connected in series to configure asingle interferometer optical switch. This embodiment demonstrated theoperation of the interferometer optical switch as a constant powerconsumption, broad band 1×2 switch. However, the interferometer opticalswitch presented in this embodiment can be used for differentapplications. Moreover, any configuration other than that described inthis embodiment can be used to configure a wide-range 1×2 optical switchor a wide-range optical switch with constant power consumption.

EIGHTH EMBODIMENT

FIG. 29 shows a configuration of the interferometer optical switch of aneighth embodiment in accordance with the present invention. The opticalswitch of the present embodiment can operate as a 1×2 switch with aPI-Loss (Path Independent Loss) configuration. Furthermore, it can carryout the switching operation independent of the wavelength band by usingthe phase generating couplers, the phase differences of the outputs ofwhich have wavelength dependence, as the optical multi/demultiplexingdevices of the interferometer optical switch, which are the basiccomponent of the present embodiment.

The circuit of the interferometer optical switch of the presentembodiment is configured by connecting a plurality of interferometeroptical switches in cascade. More specifically, using three, first tothird, interferometer optical switches 170, 171 and 172 with the samedesign values, the first interferometer optical switch 170 has its upperport at the output side connected to the lower port at the input side ofthe second interferometer optical switch 171, and has its lower port atthe output side connected to the upper port at the input side of thethird interferometer optical switch 172. In addition, the firstinterferometer optical switch 170 has its upper port at the input sideused as the input waveguide 101, the second interferometer opticalswitch 171 has its upper port at the output side used as the outputwaveguide 103 (first output port), and the third interferometer opticalswitch 172 has its upper port at the output side used as the outputwaveguide 104 (second output port).

Since the present embodiment employs three interferometer opticalswitches 170, 171 and 172 with the same design values, only the firstinterferometer optical switch 170 will be described in detail. Toachieve uniform output intensity of the interferometer optical switch ofthe present embodiment throughout the wavelength band, optimization wasperformed for the power coupling ratios of the optical couplers 151-154constituting the individual phase generating couplers 111 and 112, theoptical path length differences of the minute optical delay lines 132and 133, and the optical path length difference ΔL of the optical delayline 131 of the Mach-Zehnder interferometer. As a result of numericalcalculations, the power coupling ratios of the directional couplers 151and 152, and 153 and 154 constituting the phase generating couplers 111and 112 were set at r₁=0.2, r₂=0.2, r₃=0.2 and r₄=0.2, and the opticalpath length differences of the minute optical delay lines 132 and 133were set at ΔL₁=0.15·λc (≈0.23 μm) and ΔL₂=0.15·λc (≈0.23 μm). Inaddition, the optical path length difference of the optical delay line131 of the Mach-Zehnder interferometer was set at ΔL=0.28·λc (≈0.43 μm),and the spacing between the two optical waveguides of the optical delayline 131 was set at 200 μm. As the phase shifters 141 and 142, a thinfilm heater was used whose width was 50 μm, and length was 3 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was1.5%, and the core cross section of the optical waveguides was 4.5×4.5μm².

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module.

Next, the switching operation will be described in the case where alight is input via the input waveguide 101 of the optical switch modulefabricated, and the output port is switched from the output waveguide103 (first output port) to 104 (second output port). In the initialstate, the thin film heaters (144 and 146) are operated to change theoptical path length differences of the interferometer optical switches171 and 172 to ΔL+δΔL=0.28λc−0.50λc=−0.22λc. In this state, theinterferometer optical switch 170 of the first stage is in the barstate, and the two interferometer optical switches 171 and 172 of thesecond stage are in the cross state. Accordingly, the light input viathe input waveguide 101 is completely transmitted via the through portof the interferometer optical switch 170, and is output from the outputwaveguide 103 (first output port) of the interferometer optical switch171. On the other hand, light is not output from the output waveguide104 (second output port) because the light is cut off by the cross portof the interferometer optical switch 170 and the through port of theinterferometer optical switch 172. Next, the thin film heaters (144 and146) are turned off again, and the power is supplied to the thin filmheater (142) to change the optical path length difference of theinterferometer optical switch 170 to ΔL+δΔL=0.28λc−0.50λc=−0.22λc. Inthis state, the interferometer optical switch 170 of the first stage isplaced in the cross state, and the interferometer optical switches 171and 172 of the second stage are placed in the bar state. Accordingly,light input via the input waveguide 101 is transmitted through the crossport of the interferometer optical switch 170, and is output from theoutput waveguide 104 (second output port) of the interferometer opticalswitch 172. On the other hand, since the light is cut off by the throughport of the interferometer optical switch 170 and the cross port of theinterferometer optical switch 171, light is not output from the outputwaveguide 103 (first output port).

FIG. 30A illustrates the wavelength characteristics of the transmittancein the initial state (OFF state) output from the output waveguide 103(first output port) of the interferometer optical switch of the presentembodiment; and FIG. 30B illustrates the wavelength characteristics ofthe transmittance in a post-switching state (ON state) output from theother output waveguide 104 (second output port). In either case, a highextinction ratio equal to or greater than 45 dB is obtained over a broadwavelength band of 1.45-1.65 μm. In addition, the interferometer opticalswitch of the present embodiment has a PI-Loss configuration in which,light passes through same amount of path regardless of the output portselected. Thus, same characteristics are obtained for the first path(input from 101 and output from 103) and the second path (input from 101and output from 104).

In the present embodiment, the interferometer optical switches includingthe phase generating couplers are connected in cascade to configure asingle interferometer optical switch. This embodiment demonstrated theoperation of the interferometer optical switch as a PI-LOSS broad band1×2 switch with the same wavelength characteristics independent of theoptical path. However, the interferometer optical switch presented inthis embodiment can be used for different applications. Moreover, anyconfiguration other than that described in this embodiment can be usedto configure a wide-range 1×2 optical switch or a wide-range opticalswitch with PI-LOSS characteristics.

NINTH EMBODIMENT

FIG. 31 shows a configuration of the interferometer optical switch of aninth embodiment in accordance with the present invention. Theinterferometer optical switch of the present embodiment has on itsoptical waveguides birefringent index adjusting means, and can operateas a polarization beam switch as will be described below. Furthermore,it can carry out the switching operation independent of the wavelengthby using the phase generating couplers, the phase differences of theoutputs of which have wavelength dependence, as the opticalmulti/demultiplexing devices of the interferometer optical switch, whichare the basic component of the present embodiment.

The interferometer optical switch of the present embodiment isconfigured by connecting two interferometer optical switches 170 and 171in cascade. More specifically, using the two, the first and second,interferometer optical switches 170 and 171 with the same design values,the first interferometer optical switch 170 has its upper port at theoutput side connected to the lower port at the input side of the secondinterferometer optical switch 171. In addition, the first interferometeroptical switch 170 has its upper port at the input side used as theinput waveguide 101, the second interferometer optical switch 171 hasits upper port at the output side used as the output waveguide 103(first output port), and the first interferometer optical switch 170 hasits lower port at the output side used as the output waveguide 104(second output port).

Since the present embodiment employs the first and second interferometeroptical switches 170 and 171 with the same design values, only the firstinterferometer optical switch 170 will be described in detail. Thepresent embodiment uses as a means for implementing the phase generatingcoupler 111 or 112 an optical multi/demultiplexing device including N+1(=2) optical couplers 151 and 152, or 153 and 154, and N (=1) minuteoptical delay line 132 or 133 between the adjacent optical couplers. Asthe optical couplers 151 and 152, or 153 and 154, directional couplerseach including two optical waveguides placed side by side in closeproximity are used. In the present embodiment, the applicable wavelengthrange is set at 1.45-1.65 μm, and the design parameters are optimized insuch a manner that the power coupling ratios of the phase generatingcouplers 111 and 112 become about 0.5 throughout the wavelength bandused, and the phase difference Φ(λ) of the output agrees with theappropriate phase Ψ(λ). The optimized design parameters include thepower coupling ratios of the optical couplers 151 and 152, and 153 and154 constituting the phase generating couplers 111 and 112; the opticalpath length differences of the minute optical delay lines 132 and 133;and the optical path length difference ΔL of the optical delay line 131of the Mach-Zehnder interferometer. As a result of numericalcalculations, N was set at 1, the power coupling ratios of thedirectional couplers 151 and 152, and 153 and 154 were set at r₁=0.2,r₂=0.2, r₃=0.2 and r₄=0.2, and the optical path length differences ofthe minute optical delay lines 132 and 133 were set at ΔL₁=0.15·λc(≈0.23 μm) and ΔL₂=0.15·λc (=0.23 μm). In addition, the optimum value ofthe optical path length difference of the optical delay line 131 of theMach-Zehnder interferometer was set at ΔL=0.28·λc (≈0.43 μm) to placethe interferometer optical switch 170 in the through state in theinitial state in which the phase shifters were not driven. To facilitatethe operation of the switch as a polarization beam switch, the presentembodiment uses different design values for the optical path lengthdifferences of the optical delay lines of the Mach-Zehnderinterferometers in the first interferometer optical switch 170 and thesecond interferometer optical switch 171 in the initial state. This willbe described in a more detail later. The optical path length differenceof the first and second optical waveguides that forms the optical delayline 131 of the first interferometer optical switch 170 is set atΔL′=ΔL−0.5λc=−0.22λc (−0.34 μm), and the optical path length differenceof the first and second optical waveguides that forms the optical delayline 134 of the second interferometer optical switch 171 is set atΔL”=ΔL=0.28·λc (≈0.43 μm). The spacing between the two, the first andsecond, optical waveguides was set at 200 μm. As the phase shifters 141,142, 143 and 144, a thin film heater was used whose width was 40 μm, andlength was 5 mm.

According to the foregoing design values, the silica-based opticalwaveguide circuit was fabricated by using flame hydrolysis deposition,photolithography technique and reactive ion etching. It was fabricatedsuch that the relative refractive index of the optical waveguides was0.75% and the core cross section of the optical waveguides was 6×6 μm².

After the silica-based optical waveguide circuit was fabricated, thebirefringent index of the first optical waveguide of the optical delayline 131 of the first interferometer optical switch 170 was adjusted bythe birefringent index adjusting means 191 so that the optical pathlength difference of the TM mode became longer than the optical pathlength difference of the TE mode by 0.5λc. More specifically, since theoptical path length differences of the TE mode and TM mode after thebirefringent index adjustment were ΔnL_(TE)=ΔnL and ΔnL_(TM)=ΔnL+0.5λc,respectively, the difference was ΔnL_(TM)−ΔnL_(TE)=0.5λc. Thus, underthe assumption that the optical path length differences of the TE modeand TM mode in the initial state were ΔL′_(TE)=ΔL′ and ΔL′_(TM)=ΔL′, thebirefringent index adjustment provided ΔL′_(TE)=ΔL′+ΔnL andΔL′_(TM)=ΔL′+ΔnL+0.5λc. In the process of the birefringent indexadjustment, the optical path length of the first optical waveguidebecame longer than that of the initial state by ΔnL. Considering this,the optical path length difference of the second optical waveguide waslengthened by ΔnL by adjusting the effective refractive index of thesecond optical waveguide. Thus, the optical path length differences ofthe TE mode and TM mode were made ΔL′_(TE)=ΔL′ (=−0.22λc) andΔL′_(TM)=ΔL′+0.5λc (=0.28λc), respectively.

Next, the birefringent index of the second optical waveguide of theoptical delay line 134 of the second interferometer optical switch 171was adjusted by the birefringent index adjusting means 194 so that theoptical path length difference of the TM mode became longer than theoptical path length difference of the TE mode by 0.5λc. Morespecifically, since the optical path length differences of the TE modeand TM mode after the birefringent index adjustment were ΔnL_(TE)=ΔnLand ΔnL_(TM)=ΔnL+0.5λc, respectively, the difference wasΔnL_(TM)−ΔnL_(TE)=0.5λc. Thus, under the assumption that the opticalpath length differences of the TE mode and TM mode in the initial statewere ΔL″_(TE)=ΔL″ and ΔL″_(TM)=ΔL″, the birefringent index adjustmentprovided ΔL″_(TE)=ΔL″−ΔnL and ΔL″_(TM)=ΔL″−ΔnL−0.5λc. Here, the signswere made negative because the path length difference is represented interms of the relative optical path length of the first optical waveguidewith respect to that of the second optical waveguide. In the process ofthe birefringent index adjustment, the optical path length of the secondoptical waveguide became longer than that of the initial state by ΔnL.Considering this, the optical path length difference of the firstoptical waveguide was lengthened by ΔnL by adjusting the effectiverefractive index of the first optical waveguide. Thus, the optical pathlength differences of the TE mode and TM mode were made ΔL″_(TE)=ΔL″(=0.28λc) and ΔL″_(TM)=ΔL″−0.5λc (=−0.22λc), respectively.

As the birefringent index adjustment means 191-194, there are many meansknown such as a method of using light irradiation like laserirradiation, a method of using a thin film heater, a method of mountinga stress-applying film, a method of varying the geometry of thewaveguides, and a method of locally varying the material of thewaveguides, and any desired means can be used.

A chip on which the interferometer optical switch was formed was diced,a heatsink (not shown) was disposed under the silicon substrate 161,single mode fibers (not shown) were connected to the input/outputwaveguides 101-104, and feeder leads (not shown) were connected to thethin film heaters 141, thereby forming a two-input, two-output opticalswitch module.

Next, the switching operation will be described in the case where lightis input via the input waveguide 101 of the optical switch modulefabricated, and the polarization output from the output waveguide 103(first output port) and 104 (second output port) is switched. In theinitial OFF state, the optical path length differences of the opticaldelay lines of the first interferometer optical switch 170 areΔL′_(TE)=ΔL′ (=−0.22λc) for the TE mode, and ΔL′_(TM)=ΔL′+0.5λc(=0.28λc) for the TM mode, while the optical path length differences ofthe optical delay lines of the second interferometer optical switch 171are ΔL″_(TE)=ΔL″ (=0.28·λc) for the TE mode, and ΔL″_(TM)=ΔL″−0.5λc(=−0.22λc) for the TM mode.

For the TE mode in the OFF state, the first interferometer opticalswitch 170 is in the cross state, and the second interferometer opticalswitch 171 is in the bar state. FIG. 32A illustrates the wavelengthdependence of the transmittance of the TE mode in the OFF state of theinterferometer optical switch of the present embodiment. The TE modeinput via the input waveguide 101 is completely transmitted through thecross port of the interferometer optical switch 170, and is output fromthe output waveguide 104 (second output port). On the other hand, sinceit is cut off by the through port of the interferometer optical switch170 and the cross port of the interferometer optical switch 171, the TEmode is not output from the output waveguide 103 (first output port).

For the TM mode in the OFF state, the first interferometer opticalswitch 170 is in the bar state, and the second interferometer opticalswitch 171 is in the cross state. FIG. 32B illustrates the wavelengthdependence of the transmittance of the TM mode in the OFF state. The TMmode input via the input waveguide 101 is transmitted through thethrough port of the interferometer optical switch 170 and the cross portof the interferometer optical switch 171, and is output from the outputwaveguide 103 (first output port). On the other hand, since it is cutoff by the cross port of the interferometer optical switch 170, the TMmode is not output from the output waveguide 104 (second output port).

Next, by providing electric power, the thin film heaters 141 and 143 arebrought into the ON state. The optical path length difference of theoptical delay line 131 of the first interferometer optical switch 170 isΔL′_(TE)=ΔL′+0.5λc (=0.28λc) for the TE mode, and ΔL′_(TM)=ΔL′+1.0λc(=0.78λc) for the TM mode, while the optical path length difference ofthe optical delay line 134 of the second interferometer optical switch171 is ΔL″_(TE)=ΔL″+0.5λc (=0.78·λc) for the TE mode, and ΔL″_(TM)=ΔL″(=0.28λc) for the TM mode.

For the TE mode in the ON state, the first interferometer optical switch170 is in the bar state, and the second interferometer optical switch171 is in the cross state. FIG. 33A illustrates the wavelengthdependence of the transmittance of the TE mode in the ON state of theinterferometer optical switch of the present embodiment. The TE modeinput via the input waveguide 101 is transmitted through the throughport of the interferometer optical switch 170 and the cross port of theinterferometer optical switch 171, and is output from the outputwaveguide 103 (first output port). On the other hand, since it is cutoff by the cross port of the interferometer optical switch 170, the TEmode is not output from the output waveguide 104 (second output port).

For the TM mode in the ON state, the first interferometer optical switch170 is in the cross state, and the second interferometer optical switch171 is in the bar state. FIG. 33B illustrates the wavelength dependenceof the transmittance of the TM mode in the ON state. The TM mode inputvia the input waveguide 101 is completely transmitted through the crossport of the interferometer optical switch 170, and is output from theoutput waveguide 104 (second output port). On the other hand, since itis cut off by the through port of the interferometer optical switch 170and the cross port of the interferometer optical switch 171, the TM modeis not output from the output waveguide 103 (first output port).

As described above, the present embodiment is an example that carriesout the birefringent index adjustment of the interferometer opticalswitch having equipped with phase generating couplers, the phasedifferences of the outputs of which have wavelength dependence. Asdescribed in the present embodiment, the switch can be operated aspolarization beam switch by setting the difference of the optical pathlength difference between the TE mode and TM mode of the Mach-Zenderinterferometer delay line at a half wavelength. It is obvious that thepresent embodiment can take other forms. In addition, the birefringentindex adjustment can be used to implement an interferometer opticalswitch with small polarization dependence by setting the optical pathlength difference of the TE mode and TM mode at a same value.

OTHER EMBODIMENTS

The interferometer optical switches described in the individualembodiments in accordance with the present invention can be used as anoptical switch by itself, or can be used as components of a tap switch,a gate switch, a double gate switch or a 1×2 switch by combining aplurality of these optical switches. In addition, by using at least oneinterferometer optical switch in accordance with the present inventionas a basic component, it is possible to configure an N×N matrix switch(see FIG. 34A), a 1×N tree switch (see FIG. 34B), a 1×N tap switch, a DC(Delivery and-Coupling) switch composed of M 1×N switches and N M×1couplers, or an M×N large scale optical switch such as an ROADM(Reconfigurable OADM) switch. Furthermore, they can be combined with AWGto configure an optical add/drop multiplexing (OADM) circuit, forexample, rather than operating them only as an optical switch.

In FIG. 34A and FIG. 34B, reference numerals 180-1 a-180-8 a eachdesignate an input waveguide, and 181-1 b-181-8 b each designate anoutput waveguide, and the reference numeral 182 designates a basiccomponent of the optical switch, 183 designates a cross of the basiccomponent of the optical switch, 184 designates a 1×2 switch, and 185designates a gate switch.

Although the individual embodiments show applications to theinterferometer optical switches or variable optical attenuators,including the polarization beam switch, polarization beam splitter andpolarization beam coupler, the present invention is applicable to anydesired circuits. Furthermore, the interferometer optical switch andvariable optical attenuator in accordance with the present invention canbe combined to be functioned as a single optical circuit. In addition,although the individual embodiments in accordance with the presentinvention show examples applied to the Mach-Zehnder interferometerhaving only one optical delay line, this is not essential. For example,as for a configuration having two or more optical delay lines, a varietyof wavelength insensitive optical waveguide circuits can be obtained byapplying the same principle. For example, the principle of the presentinvention is applicable to a variety of optical waveguide circuits suchas lattice-form filters, multiple beam interference filters,transversal-form filters, Michelson interferometer filters, Fabry-Perotinterferometer filters, and ring resonator filters. Here, the opticalpath length difference described in the individual embodiments refers tothe optical path difference between the optical waveguides constitutingthe optical delay line. The optical path difference considers therefractive index or birefringent index of the optical waveguides withwavelength dependence. In this way, a variety of wavelength insensitiveoptical waveguide circuits can be implemented by using the phasedifference of the output of the optical multi/demultiplexing device, andby making wavelength insensitive the phase difference caused by the pathlength difference of the optical delay line. It is obvious that thepresent invention can eliminate not only the wavelength dependence, butalso the frequency dependence.

The foregoing embodiments each fabricated the interferometer opticalswitch and variable optical attenuator through the process asillustrated in FIGS. 35A-35E using silica-based optical waveguidesformed on the silicon substrate. More specifically, on a siliconsubstrate 161, an undercladding glass soot 162 mainly composed of SiO₂and a core glass soot 163 composed of SiO₂ doped with GeO₂ weredeposited by flame hydrolysis deposition (FIG. 35A). Subsequently, theywere made to increase the transparency of glass at a high temperaturebeyond degrees 1000 Celsius. During the process, the glass depositionwas carried out so that the undercladding glass layer 164 and the coreglass 165 became a designed thickness (FIG. 35B). Subsequently, anetching mask 166 was formed on the core glass 165 using photolithographytechnique (FIG. 35C), followed by patterning the core glass 165 byreactive ion etching (FIG. 35D). After removing the etching mask 166, anovercladding glass 167 was formed by flame hydrolysis deposition, again.To the overcladding glass 167, dopants such as B₂O₃ or P₂O₃ were added,and the glass transition temperature was lowered so that theovercladding glass 167 penetrated into the narrow gaps between the coreglass 165 and core glass 165 (FIG. 35E). Furthermore, on the surface ofthe overcladding glass 167, the thin film heaters (not shown) andelectric wiring (not shown) were patterned.

The optical modules described in the individual embodiments wereconstructed as follows (see FIG. 36). Specifically, as for the opticalmodule, in a high thermal conductivity module 701, a Peltier holdingplate 702 was fixed with mounting screws 703, and a Peltier element anda temperature sensor (thermocouple) (not shown) were disposed close toeach other in a concave formed by digging the Peltier holding plate 702.Directly above the Peltier element and temperature sensor, a chip 704including the interferometer optical switch or variable opticalattenuator described in the individual embodiments was disposed. Atedges of the chip 704, glass plates 705 were fastened with an adhesivein such a manner that they make optical coupling with fiber blocks 707holding fibers 706. The fibers 706 were joined to the concaves at edgesof the module 701 with adiabatic elastic adhesive 708, and were furtherheld in such a manner that fiber boots 710 having fiber cords 709 wereburied in the module 701. The chip 704 is joined to the Peltier holdingplate with the adiabatic elastic adhesive 708. Finally, a cover was fitby screws to shield them, thereby assembling the optical module inaccordance with the present invention. Here, the cover and the screwsare not shown, and it is only an example of a module. In the individualembodiments in accordance with the present invention, although the inputwaveguide and output waveguide are drawn out from the different edges ofthe chip, it is obvious that such a circuit layout is also possible inwhich they are placed on the same edge. In this case, only a singlefiber block is enough to connect the input waveguide and outputwaveguide to the fibers.

The circuit in accordance with the present invention can be fabricatedas separate independent chips. In this case, they can be integrated intoa single chip by directly interconnecting the chips, or they can bearranged into an optical module by optically coupling the plurality ofchips. In addition, it is also possible to fabricate separate opticalmodules for individual chips, followed by coupling the optical modulesthrough fibers. Furthermore, an optical module can also be fabricated inwhich two or more chips are held on the Peltier holding plate in asingle module.

As for the form of the interferometer optical switch or variable opticalattenuator in accordance with the present invention, it does not dependon the types, geometry, materials, refractive index or fabricationmethod of the optical waveguides. For example, as for the material ofthe waveguides, it may be polyimide, silicon, semiconductor, LiNbO₂ orthe like, and the substrate material may be quartz. In addition, thepresent invention is applicable even when the fabrication method is aspin coating method, a sol-gel method, a sputtering method, a CVDmethod, an ion diffusion method, or ion beam direct patterning method.Furthermore, although the individual embodiments in accordance with thepresent invention use square optical waveguides, any desired geometrysuch as a rectangle, a polygon, a circle can be used. For example, thecore width of some part of the optical waveguide can be changed so thatits refractive index difference is different from that of the remainingoptical waveguide. In addition, the optical waveguide can be providedwith a stress to alter the value of the refractive index. Furthermore,although light was transmitted through silica-based optical waveguidesin the embodiments described above, light can travel through differentmaterials. For example, the optical waveguide can contain a materialsuch as a silicon resin, or a polyimide wave plate. Besides, a varietyof temperature compensation methods or polarization dependence reductionmethods can be applied.

In addition by using a light irradiation method such as laserirradiation, or a local heat treatment method with a thin film heater,it is possible to locally vary the refractive index of the opticalwaveguides in order to adjust the optical path length difference or thecoupling characteristics or the phase characteristics of the opticalmulti/demultiplexing device. Although the thermooptic effect activatedby the thin film heater is used for the switching operation, this is notessential. For example, a light irradiation can be used, or theelectro-optic effect, or magnetooptic effect can also be used. It isobvious that the geometry of that region is arbitrary.

Furthermore, the interferometer optical switch or variable opticalattenuator in accordance with the present invention is not limited tothe planar optical waveguides. For example, the optical waveguides maybe configured by using stacked optical waveguides or optical fibers, orby combining a plurality of types of optical waveguides such as planaroptical waveguides and optical fibers. In addition, a grating can beformed on the optical waveguides. Moreover, optical waveguides can besplit or segmented. It is obvious that the interferometer optical switchand variable optical attenuator in accordance with the present inventionis not limited to the optical waveguides, but an interference circuitcan be constructed with a spatial optical system that propagates lightthrough space. For example, the spatial optical system can be configuredby a semi-transparent mirror, a total reflection mirror and amultilayer. By thus using the spatial optical system, same advantagescan be achieved as in the case where the circuit is configured withoptical waveguides. Furthermore, the interferometer optical switch andvariable optical attenuator of the individual embodiments describedabove are one of the configurations in accordance with the presentinvention, and the present invention is not limited to theseconfigurations.

The foregoing embodiments use an optical multi/demultiplexing devicecomposed of N+1 optical couplers and N optical delay lines to configurea phase generating coupler, but such a configuration is one of theexamples for implementing a phase generating coupler. For example, it ispossible to use other filter architectures for opticalmulti/demultiplexing devices such as a multiple beam interferencefilter, a transversal-form filter, a Michelson interferometer filter, aFabry-Perot interferometer filter and a ring resonator filter.Furthermore, as the optical coupler constituting the phase generatingcoupler, or as the optical multi/demultiplexing device, it is possibleto use any desired types such as a multimode interferometer, a variablecoupler, an X branching coupler and a Y branching coupler besides thedirectional coupler described in the individual embodiments inaccordance with the present invention, and to use their combinations.Moreover, as for the values and calculation methods of the powercoupling ratios of the optical multi/demultiplexing devices, or thevalues of the path length difference of the optical delay line, they arealso one of the examples. It is desirable to design the circuitvariables according to the applications. In addition, in the case wherethe phase generating couplers have a plurality of configuration methodsand optimum values, the best form can be selected considering the size,fabrication tolerance, excess loss and the like.

Finally, although the present invention has been described in detailwith respect to preferred embodiments and their variations, theembodiments in accordance with the present invention are not limited tothose examples. A variety of variations such as replacement,modifications, additions, increase or decrease in the number, changes inthe geometry of the components are all included in the embodiments inaccordance with the present invention as long as they fall within thescope of the claims.

INDUSTRIAL APPLICABILITY

With the arrival of a multimedia era, implementation of networks thatcan handle enormous amount of information effectively has become a greatproblem. To construct networks having a flexible and effectiveconfiguration from now on including the replacement of existing coppercables with optical fiber in the access systems interconnectingindividual homes with networks via optical fibers, it is essential toadopt optical wavelength division multiplexing (WDM) technique. Theoptical components have an extensive band of a few terahertz originally,and power networks utilize only very small part of the band. If thewavelength division multiplexing can make the band available by dividingit, networks will be implemented which can increase the capacity andhandle diverse information essential for multimedia communication inhigh volume and with ease.

Recently, the optical cross connect systems and optical add/dropmultiplexing systems using the optical switches have been a key deviceof the WDM technique, and a large demand for them can be expected. Theinterferometer optical switch and variable optical attenuator inaccordance with the present invention are applicable not only in theoptical systems, but also as an optical switch element.

1. An interferometer optical switch comprising an optical waveguidecircuit including: a first optical multi/demultiplexing device; anoptical delay line including two optical waveguides connected to saidfirst optical multi/demultiplexing device; a second opticalmulti/demultiplexing device connected to said optical delay line; one ormore input waveguides connected to said first opticalmulti/demultiplexing device; one or more output waveguides connected tosaid second optical multi/demultiplexing device; and a phase shifterinstalled in said optical delay line, and wherein at least one of saidfirst optical multi/demultiplexing device and said second opticalmulti/demultiplexing device is a phase generating coupler, whichproduces a wavelength-dependent phase difference.
 2. The interferometeroptical switch as claimed in claim 1, wherein assuming that λ is thewavelength, 2πφ₁(λ) is the phase produced by the first opticalmulti/demultiplexing device, 2πφΔ_(L)(λ) is the phase difference of theoptical delay line with an optical path length difference of ΔL, and2πφ₂(λ) is the phase produced by the second optical multi/demultiplexingdevice, the phase produced by the first and second opticalmulti/demultiplexing device and the optical path length difference ΔL isset such that the sum of the phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}becomes wavelength insensitive.
 3. The interferometer optical switch asclaimed in claim 2, wherein the sum of the phase difference φ₁(λ) of theoutput of said first optical multi/demultiplexing device and the phasedifference φ₂(λ) of the output of said second opticalmulti/demultiplexing device equals ΔL/λ+m/2 (m is an integer).
 4. Theinterferometer optical switch as claimed in claim 2, wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at(2m′+1)·π (m′ is an integer), and the power coupling ratio of said firstoptical multi/demultiplexing device and the power coupling ratio of saidsecond optical multi/demultiplexing device are made equal throughout anentire wavelength region.
 5. The interferometer optical switch asclaimed in claim 2, wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of thethree phase differences is set at 2m′·π (m′ is an integer), and the sumof the power coupling ratio of said first optical multi/demultiplexingdevice and the power coupling ratio of said second opticalmulti/demultiplexing device is made unity.
 6. The interferometer opticalswitch as claimed in claim 1, wherein assuming that λ is the wavelength,2πφ₁(λ) is the phase produced by the first optical multi/demultiplexingdevice, 2πφΔ_(L)(λ) is the phase difference of the optical delay linewith an optical path length difference of ΔL, and 2πφ₂(λ) is the phaseproduced by the second optical multi/demultiplexing device, the sum ofthe phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that theoutput intensity of said optical waveguide circuit becomes uniform withrespect to wavelength.
 7. The interferometer optical switch as claimedin claim 1, wherein said phase generating coupler is configured byconnecting optical couplers and optical delay lines.
 8. Theinterferometer optical switch as claimed in claim 7, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the phase produced by the first and secondoptical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive.
 9. Theinterferometer optical switch as claimed in claim 8, wherein the sum ofthe phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 10. The interferometer optical switch as claimed in claim4, wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences is set at (2m′+1)·π (m′ is an integer), and the powercoupling ratio of said first optical multi/demultiplexing device and thepower coupling ratio of said second optical multi/demultiplexing deviceare made equal.
 11. The interferometer optical switch as claimed inclaim 8, wherein the sum 2π{φ₁(λ)+φΔL(λ)+φ₂(λ)} of the three phasedifferences is set at 2m′·π (m′ is an integer), and the sum of the powercoupling ratio of said first optical multi/demultiplexing device and thepower coupling ratio of said second optical multi/demultiplexing deviceis made unity.
 12. The interferometer optical switch as claimed in claim7, wherein assuming that λ is the wavelength, 2πφ₁(λ) is the phaseproduced by the first optical multi/demultiplexing device, 2πφΔ_(L)(λ)is the phase difference of the optical delay line with an optical pathlength difference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.13. The interferometer optical switch as claimed in claim 7, whereinsaid phase generating coupler comprises N+1 optical couplers (N is anatural number), and N optical delay lines that connects adjacentoptical couplers of said N+1 optical couplers.
 14. The interferometeroptical switch as claimed in claim 13, wherein assuming that λ is thewavelength, 2πφ₁(λ) is the phase produced by the first opticalmulti/demultiplexing device, 2πφΔ_(L)(λ) is the phase difference of theoptical delay line with an optical path length difference of ΔL, and2πφ₂(λ) is the phase produced by the second optical multi/demultiplexingdevice, the phase produced by the first and second opticalmulti/demultiplexing device and the optical path length difference ΔL isset such that the sum of the phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}becomes wavelength insensitive.
 15. The interferometer optical switch asclaimed in claim 14, wherein the sum of the phase difference φ₁(λ) ofthe output of said first optical multi/demultiplexing device and thephase difference φ₂(λ) of the output of said second opticalmulti/demultiplexing device equals ΔL/λ+m/2 (m is an integer).
 16. Theinterferometer optical switch as claimed in claim 14, wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at(2m′+1)·π (m′ is an integer), and the power coupling ratio of said firstoptical multi/demultiplexing device and the power coupling ratio of saidsecond optical multi/demultiplexing device are made equal.
 17. Theinterferometer optical switch as claimed in claim 14, wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at 2m′·π(m′ is an integer), and the sum of the power coupling ratio of saidfirst optical multi/demultiplexing device and the power coupling ratioof said second optical multi/demultiplexing device is made unity. 18.The interferometer optical switch as claimed in claim 13, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.19. The interferometer optical switch as claimed in claim 7, wherein oneof said first optical multi/demultiplexing device and said secondoptical multi/demultiplexing device is an optical coupler with a phasedifference 2πφ_(c) (constant), and the other is a phase generatingcoupler that is composed of two optical couplers and an optical delayline placed between said two optical couplers, and has a phasedifference 2πφ(λ), and wherein assuming that ΔL is the optical pathlength difference of the optical delay line, and m is an integer, thenthe power coupling ratios of the two optical couplers constituting saidphase generating coupler, and the optical path length difference of theoptical delay line are set to satisfyφ(λ)=ΔL/λ+m/2−φ_(c)  (11).
 20. The interferometer optical switch asclaimed in claim 19, wherein assuming that λ is the wavelength, 2πφ₁(λ)is the phase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device and the optical pathlength difference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔL(λ)+φ₂(λ)} becomes wavelength insensitive, and wherein thesum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at(2m′+1)·π (m′ is an integer), and the power coupling ratio of said firstoptical multi/demultiplexing device and the power coupling ratio of saidsecond optical multi/demultiplexing device are made equal throughout anentire wavelength region.
 21. The interferometer optical switch asclaimed in claim 19, wherein assuming that λ is the wavelength, 2πφ₁(λ)is the phase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device and the optical pathlength difference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, and whereinthe sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is setat 2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.22. The interferometer optical switch as claimed in claim 19, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.23. The interferometer optical switch as claimed in claim 7, whereinsaid first optical multi/demultiplexing device and said second opticalmulti/demultiplexing device are both a phase generating couplercomprising two optical couplers and a single optical delay line placedbetween said two optical couplers, and wherein power coupling ratios ofthe two optical couplers and an optical path length difference of theoptical delay line that constitutes the first and second opticalmulti/demultiplexing device are set such that the sum of the phasedifference 2πφ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference 2πφ₂(λ) of theoutput of said second optical multi/demultiplexing device satisfiesφ₁(λ)+φ₂(λ)=ΔL/λ+m/2  (12) where ΔL is the optical path lengthdifference of said optical delay line, and m is an integer.
 24. Theinterferometer optical switch as claimed in claim 23, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device and the optical path length difference ΔL isset such that the sum of the phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}becomes wavelength insensitive, and wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at(2m′+1)·π (m′ is an integer), and the power coupling ratio of said firstoptical multi/demultiplexing device and the power coupling ratio of saidsecond optical multi/demultiplexing device are made equal throughout anentire wavelength region.
 25. The interferometer optical switch asclaimed in claim 23, wherein assuming that λ is the wavelength, 2πφ₁(λ)is the phase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device and the optical pathlength difference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, the sum2π{φ₁(λ)+φΔ_(L(λ)+φ) ₂(λ)} of the three phase differences is set at2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.26. The interferometer optical switch as claimed in claim 23, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.27. The interferometer optical switch as claimed in claim 7, whereinsaid first optical multi/demultiplexing device and said second opticalmulti/demultiplexing device are both a phase generating couplercomprising N+1 optical couplers (N is a natural number), and N opticaldelay lines each of which is composed of a first and second opticalwaveguides, and which connects adjacent optical couplers of the N+1optical couplers, and wherein the sum of the optical path lengthsatisfies either Σl_(1,1)>Σl_(2,1) and Σl_(1,2)>Σl_(2,2)) or(Σl_(2,1)>Σl_(1,1) and Σl_(2,2)>Σl_(1,2)), where Σl_(1,1) is the sum ofoptical path lengths of the first optical waveguide constituting the Noptical delay lines of said first optical multi/demultiplexing device,Σl_(2,1) is the sum of optical path lengths of the second opticalwaveguide, Σl_(1,2) is the sum of optical path lengths of the firstoptical waveguide constituting the N optical delay lines of said secondoptical multi/demultiplexing device, and Σl_(2,2) is the sum of opticalpath lengths of the second optical waveguides constituting the N opticaldelay lines of said second optical multi/demultiplexing device.
 28. Theinterferometer optical switch as claimed in claim 27, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the phase produced by the first and secondoptical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive.
 29. Theinterferometer optical switch as claimed in claim 28, wherein the sum ofthe phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 30. The interferometer optical switch as claimed in claim28, wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences is set at (2m′+1)·π (m′ is an integer), and the powercoupling ratio of said first optical multi/demultiplexing device and thepower coupling ratio of said second optical multi/demultiplexing deviceare made equal.
 31. The interferometer optical switch as claimed inclaim 28, wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences is set at 2m′·π (m′ is an integer), and the sum of the powercoupling ratio of said first optical multi/demultiplexing device and thepower coupling ratio of said second optical multi/demultiplexing deviceis made unity.
 32. The interferometer optical switch as claimed in claim27, wherein assuming that λ is the wavelength, 2πφ₁(λ) is the phaseproduced by the first optical multi/demultiplexing device, 2πφΔ_(L)(λ)is the phase difference of the optical delay line with an optical pathlength difference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.33. The interferometer optical switch as claimed in claim 27, whereinthe power coupling ratios of the N+1 optical couplers of said firstoptical multi/demultiplexing device are made equal to the power couplingratios of the N+1 optical couplers of said second opticalmulti/demultiplexing device.
 34. The interferometer optical switch asclaimed in claim 33, wherein the sum of the phase difference φ₁(λ) ofthe output of said first optical multi/demultiplexing device and thephase difference φ₂(λ) of the output of said second opticalmulti/demultiplexing device equals ΔL/λ+m/2 (m is an integer), whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, and whereinthe sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is setat (2m′+1)·π (m′ is an integer), and the power coupling ratio of saidfirst optical multi/demultiplexing device and the power coupling ratioof said second optical multi/demultiplexing device are made equalthroughout an entire wavelength region.
 35. The interferometer opticalswitch as claimed in claim 33, wherein the sum of the phase differenceφ₁(λ) of the output of said first optical multi/demultiplexing deviceand the phase difference φ₂(λ) of the output of said second opticalmulti/demultiplexing device equals ΔL/λ+m/2 (m is an integer); whereinassuming that λ is the wavelength, 2πφ1(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, and whereinthe sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is setat 2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.36. The interferometer optical switch as claimed in claim 33, whereinassuming that optical wavelength is λ, a phase difference between lightoutput from said first optical multi/demultiplexing device is 2πφ₁(λ), aphase difference caused by an optical path length difference ΔL of saidoptical delay line is 2πφΔ_(L)(λ), and a phase difference between lightoutput from said second optical multi/demultiplexing device is 2πφ₂(λ),then the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences isset such that output intensity of said optical waveguide circuit becomesconstant for the wavelength λ.
 37. The interferometer optical switch asclaimed in claim 7, wherein said first optical multi/demultiplexingdevice and said second optical multi/demultiplexing device each consistof a phase generating coupler including N+1 optical couplers (N is anatural number), and N optical delay lines sandwiched between adjacentsaid optical couplers of said N+1 optical couplers, and wherein thepower coupling ratios of the N+1 optical couplers of said first opticalmulti/demultiplexing device are made equal to the power coupling ratiosof the N+1 optical couplers of said second optical multi/demultiplexingdevice.
 38. The interferometer optical switch as claimed in claim 37,wherein assuming that λ is the wavelength, 2πφ₁(λ) is the phase producedby the first optical multi/demultiplexing device, 2πφΔ_(L)(λ) is thephase difference of the optical delay line with an optical path lengthdifference of ΔL, and πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the phase produced by the first and secondoptical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive.
 39. Theinterferometer optical switch as claimed in claim 38, wherein the sum ofthe phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 40. The interferometer optical switch as claimed in claim38, wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences is set at (2m′+1)·π (m′ is an integer), and the powercoupling ratio of said first optical multi/demultiplexing device and thepower coupling ratio of said second optical multi/demultiplexing deviceare made equal.
 41. The interferometer optical switch as claimed inclaim 38, wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phasedifferences is set at 2m′·π (m′ is an integer), and the sum of the powercoupling ratio of said first optical multi/demultiplexing device and thepower coupling ratio of said second optical multi/demultiplexing deviceis made unity.
 42. The interferometer optical switch as claimed in claim37, wherein assuming that λ is the wavelength, 2πφ₁(λ) is the phaseproduced by the first optical multi/demultiplexing device, 2πφΔ_(L)(λ)is the phase difference of the optical delay line with an optical pathlength difference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.43. A variable optical attenuator that uses the interferometer opticalswitch as defined in claim 1 wherein, the output intensity is varied.44. The variable optical attenuator as claimed in claim 43, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the phase produced by the first andsecond optical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive.
 45. Thevariable optical attenuator as claimed in claim 44, wherein the sum ofthe phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 46. The variable optical attenuator as claimed in claim 44,wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differencesis set at (2m′+1)·π (m′ is an integer), and the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device are made equalthroughout an entire wavelength region.
 47. The variable opticalattenuator as claimed in claim 44, wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at 2m′·π(m′ is an integer), and the sum of the power coupling ratio of saidfirst optical multi/demultiplexing device and the power coupling ratioof said second optical multi/demultiplexing device is made unity. 48.The variable optical attenuator as claimed in claim 43, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.49. The variable optical attenuator as claimed in claim 43, wherein saidphase generating coupler is configured by connecting optical couplersand optical delay lines.
 50. The variable optical attenuator as claimedin claim 49, wherein assuming that λ is the wavelength, 2πφ₁(λ) is thephase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device, the phase produced bythe first and second optical multi/demultiplexing device and the opticalpath length difference ΔL is set such that the sum of the phasedifference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive. 51.The variable optical attenuator as claimed in claim 50, wherein the sumof the phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 52. The variable optical attenuator as claimed in claim 50,wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differencesis set at (2m′+1)·π (m′ is an integer), and the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device are made equal.53. The variable optical attenuator as claimed in claim 50, wherein thesum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.54. The variable optical attenuator as claimed in claim 49, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.55. The variable optical attenuator as claimed in claim 49, wherein saidphase generating coupler comprises N+1 optical couplers (N is a naturalnumber), and N optical delay lines that connects adjacent opticalcouplers of said N+1 optical couplers.
 56. The variable opticalattenuator as claimed in claim 55, wherein assuming that λ is thewavelength, 2πφ₁(λ) is the phase produced by the first opticalmulti/demultiplexing device, 2πφΔ_(L)(λ) is the phase difference of theoptical delay line with an optical path length difference of ΔL, and2πφ₂(λ) is the phase produced by the second optical multi/demultiplexingdevice, the phase produced by the first and second opticalmulti/demultiplexing device and the optical path length difference ΔL isset such that the sum of the phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}becomes wavelength insensitive.
 57. The variable optical attenuator asclaimed in claim 56, wherein the sum of the phase difference φ₁(λ) ofthe output of said first optical multi/demultiplexing device and thephase difference φ₂(λ) of the output of said second opticalmulti/demultiplexing device equals ΔL/λ+m/2 (m is an integer).
 58. Thevariable optical attenuator as claimed in claim 56, wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at(2m′+1)·π (m′ is an integer), and the power coupling ratio of said firstoptical multi/demultiplexing device and the power coupling ratio of saidsecond optical multi/demultiplexing device are made equal.
 59. Thevariable optical attenuator as claimed in claim 56, wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at 2m′·π(m′ is an integer), and the sum of the power coupling ratio of saidfirst optical multi/demultiplexing device and the power coupling ratioof said second optical multi/demultiplexing device is made unity. 60.The variable optical attenuator as claimed in claim 55, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.61. The variable optical attenuator as claimed in claim 49, wherein oneof said first optical multi/demultiplexing device and said secondoptical multi/demultiplexing device is an optical coupler with a phasedifference 2πφ_(c) (constant), and the other is a phase generatingcoupler that is composed of two optical couplers and an optical delayline placed between said two optical couplers, and has a phasedifference 2πφ(λ), and wherein assuming that ΔL is the optical pathlength difference of the optical delay line, and m is an integer, thenthe power coupling ratios of the two optical couplers constituting saidphase generating coupler, and the optical path length difference of theoptical delay line are set to satisfyφ(λ)=ΔL/λ+m/2−φ_(c)  (11).
 62. The variable optical attenuator asclaimed in claim 61, wherein. assuming that λ is the wavelength, 2πφ₁(λ)is the phase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device and the optical pathlength difference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, and whereinthe sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is setat (2m′+1)·π (m′ is an integer), and the power coupling ratio of saidfirst optical multi/demultiplexing device and the power coupling ratioof said second optical multi/demultiplexing device are made equalthroughout an entire wavelength region.
 63. The variable opticalattenuator as claimed in claim 61, wherein assuming that λ is thewavelength, 2πφ₁(λ) is the phase produced by the first opticalmulti/demultiplexing device, 2πφΔ_(L)(λ) is the phase difference of theoptical delay line with an optical path length difference of ΔL, and2πφ₂(λ) is the phase produced by the second optical multi/demultiplexingdevice and the optical path length difference ΔL is set such that thesum of the phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelengthinsensitive, and wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the threephase differences is set at 2m′·π (m′ is an integer), and the sum of thepower coupling ratio of said first optical multi/demultiplexing deviceand the power coupling ratio of said second optical multi/demultiplexingdevice is made unity.
 64. The variable optical attenuator as claimed inclaim 61, wherein assuming that λ is the wavelength, 2πφ₁(λ) is thephase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device, the sum of the phasedifference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the outputintensity of said optical waveguide circuit becomes uniform with respectto wavelength.
 65. The variable optical attenuator as claimed in claim49, wherein said first optical multi/demultiplexing device and saidsecond optical multi/demultiplexing device are both a phase generatingcoupler comprising two optical couplers and a single optical delay lineplaced between said two optical couplers, and wherein power couplingratios of the two optical couplers and an optical path length differenceof the optical delay line that constitutes the first and second opticalmulti/demultiplexing device are set such that the sum of the phasedifference 2πφ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference 2πφ₂(λ) of theoutput of said second optical multi/demultiplexing device satisfiesφ₁(λ)+φ₂(λ)=ΔL/λ+m/2  (12) where ΔL is the optical path lengthdifference of said optical delay line, and m is an integer.
 66. Thevariable optical attenuator as claimed in claim 65, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device and the optical path length difference ΔL isset such that the sum of the phase difference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)}becomes wavelength insensitive, and wherein the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at(2m′+1)·π (m′ is an integer), and the power coupling ratio of said firstoptical multi/demultiplexing device and the power coupling ratio of saidsecond optical multi/demultiplexing device are made equal throughout anentire wavelength region.
 67. The variable optical attenuator as claimedin claim 65, wherein assuming that λ is the wavelength, 2πφ₁(λ) is thephase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device and the optical pathlength difference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, the sum2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at 2m′·π(m′ is an integer), and the sum of the power coupling ratio of saidfirst optical multi/demultiplexing device and the power coupling ratioof said second optical multi/demultiplexing device is made unity. 68.The variable optical attenuator as claimed in claim 65, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.69. The variable optical attenuator as claimed in claim 49, wherein saidfirst optical multi/demultiplexing device and said second opticalmulti/demultiplexing device are both a phase generating couplercomprising N+1 optical couplers (N is a natural number), and N opticaldelay lines each of which is composed of a first and second opticalwaveguides, and which connects adjacent optical couplers of the N+1optical couplers, and wherein the sum of the optical path lengthsatisfies either (Σl_(1,1)>Σl_(2,1) and Σl_(1,2)>Σl_(2,2)), or(Σl_(2,1)>Σl_(1,1), and Σl_(2,2)>Σl_(1,2)), where Σl_(1,1) is the sum ofoptical path lengths of the first optical waveguide constituting the Noptical delay lines of said first optical multi/demultiplexing device,Σl_(2,1) is the sum of optical path lengths of the second opticalwaveguide, Σl_(1,2) is the sum of optical path lengths of the firstoptical waveguide constituting the N optical delay lines of said secondoptical multi/demultiplexing device, and Σl_(2,2) is the sum of opticalpath lengths of the second optical waveguides constituting the N opticaldelay lines of said second optical multi/demultiplexing device.
 70. Thevariable optical attenuator as claimed in claim 69, wherein assumingthat λ is the wavelength, 2πφ₁(λ) is the phase produced by the firstoptical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phase differenceof the optical delay line with an optical path length difference of ΔL,and 2πφ₂(λ) is the phase produced by the second opticalmulti/demultiplexing device, the phase produced by the first and secondoptical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive.
 71. Thevariable optical attenuator as claimed in claim 70, wherein the sum ofthe phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 72. The variable optical attenuator as claimed in claim 70,wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differencesis set at (2m′+1)·π (m′ is an integer), and the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device are made equal.73. The variable optical attenuator as claimed in claim 70, wherein thesum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.74. The variable optical attenuator as claimed in claim 69, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.75. The variable optical attenuator as claimed in claim 69, wherein thepower coupling ratios of the N+1 optical couplers of said first opticalmulti/demultiplexing device are made equal to the power coupling ratiosof the N+1 optical couplers of said second optical multi/demultiplexingdevice.
 76. The variable optical attenuator as claimed in claim 75,wherein the sum of the phase difference φ₁(λ) of the output of saidfirst optical multi/demultiplexing device and the phase difference φ₂(λ)of the output of said second optical multi/demultiplexing device equalsΔL/λ+m/2 (m is an integer), wherein assuming that λ is the wavelength,2πφ₁(λ) is the phase produced by the first optical multi/demultiplexingdevice, 2πφΔ_(L)(λ) is the phase difference of the optical delay linewith an optical path length difference of ΔL, and 2πφ₂(λ) is the phaseproduced by the second optical multi/demultiplexing device and theoptical path length difference ΔL is set such that the sum of the phasedifference 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, andwherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differencesis set at (2m′+1)·π (m′ is an integer), and the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device are made equalthroughout an entire wavelength region.
 77. The variable opticalattenuator as claimed in claim 75, wherein the sum of the phasedifference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer); wherein assuming that λ is the wavelength, 2πφ₁(λ) is thephase produced by the first optical multi/demultiplexing device,2πφΔ_(L)(λ) is the phase difference of the optical delay line with anoptical path length difference of ΔL, and 2πφ₂(λ) is the phase producedby the second optical multi/demultiplexing device and the optical pathlength difference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive, and whereinthe sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is setat 2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.78. The variable optical attenuator as claimed in claim 75, whereinassuming that optical wavelength is λ, a phase difference between lightoutput from said first optical multi/demultiplexing device is 2πφ₁(λ), aphase difference caused by an optical path length difference ΔL of saidoptical delay line is 2πφΔ_(L)(λ), and a phase difference between lightoutput from said second optical multi/demultiplexing device is 2πφ₂(λ),then the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences isset such that output intensity of said optical waveguide circuit becomesconstant for the wavelength λ.
 79. The variable optical attenuator asclaimed in claim 49, wherein said first optical multi/demultiplexingdevice and said second optical multi/demultiplexing device each consistof a phase generating coupler including N+1 optical couplers (N is anatural number), and N optical delay lines sandwiched between adjacentsaid optical couplers of said N+1 optical couplers, and wherein thepower coupling ratios of the N+1 optical couplers of said first opticalmulti/demultiplexing device are made equal to the power coupling ratiosof the N+1 optical couplers of said second optical multi/demultiplexingdevice.
 80. The variable optical attenuator as claimed in claim 79,wherein assuming that λ is the wavelength, 2πφ₁(λ) is the phase producedby the first optical multi/demultiplexing device, 2πφΔ_(L)(λ) is thephase difference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the phase produced by the first andsecond optical multi/demultiplexing device and the optical path lengthdifference ΔL is set such that the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} becomes wavelength insensitive.
 81. Thevariable optical attenuator as claimed in claim 80, wherein the sum ofthe phase difference φ₁(λ) of the output of said first opticalmulti/demultiplexing device and the phase difference φ₂(λ) of the outputof said second optical multi/demultiplexing device equals ΔL/λ+m/2 (m isan integer).
 82. The variable optical attenuator as claimed in claim 80,wherein the sum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differencesis set at (2m′+1)·π (m′ is an integer), and the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device are made equal.83. The variable optical attenuator as claimed in claim 80, wherein thesum 2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} of the three phase differences is set at2m′·π (m′ is an integer), and the sum of the power coupling ratio ofsaid first optical multi/demultiplexing device and the power couplingratio of said second optical multi/demultiplexing device is made unity.84. The variable optical attenuator as claimed in claim 79, whereinassuming that λ is the wavelength, 2πφ₁(λ) is the phase produced by thefirst optical multi/demultiplexing device, 2πφΔ_(L)(λ) is the phasedifference of the optical delay line with an optical path lengthdifference of ΔL, and 2πφ₂(λ) is the phase produced by the secondoptical multi/demultiplexing device, the sum of the phase difference2π{φ₁(λ)+φΔ_(L)(λ)+φ₂(λ)} is set such that the output intensity of saidoptical waveguide circuit becomes uniform with respect to wavelength.85. An interferometer optical switch comprising a plurality ofinterferometer optical switches as defined in claim 1 connected incascade.
 86. A variable optical attenuator comprising a plurality ofvariable optical attenuators as defined in claim 43 connected incascade.
 87. An interferometer optical switch comprising an opticalcircuit having a plurality of interferometer optical switches as definedin claim 1 connected in cascade, wherein a first interferometer opticalswitch having two output waveguides; one of the said output waveguidesis connected to the input waveguide of a second interferometer opticalswitch; the other output waveguide of said first interferometer opticalswitch is used as the second output port of said optical circuit; theinput waveguide of said first interferometer optical switch is used asthe input port of said optical circuit; and the output waveguide of saidsecond interferometer optical switch is used as the first output port ofsaid optical circuit.
 88. A variable optical attenuator comprising anoptical circuit having a plurality of variable optical attenuators asdefined in claim 43 connected in cascade, wherein a first interferometeroptical switch having two output waveguides; one of the said outputwaveguides is connected to the input waveguide of a secondinterferometer optical switch; the other output waveguide of said firstinterferometer optical switch is used as the second output port of saidoptical circuit; the input waveguide of said first interferometeroptical switch is used as the input port of said optical circuit; andthe output waveguide of said second interferometer optical switch isused as the first output port of said optical circuit.
 89. Aninterferometer optical switch comprising an optical circuit having aplurality of interferometer optical switches as defined in claim 1connected in cascade, wherein a first interferometer optical switchhaving two output waveguides; one of the said output waveguides isconnected to the input waveguide of a second interferometer opticalswitch; the other output waveguide of said first interferometer opticalswitch is connected to the input waveguide of a third interferometeroptical switch; the input waveguide of said first interferometer opticalswitch is used as the input port of said optical circuit; the outputwaveguide of said second interferometer optical switch is used as thefirst output port of said optical circuit; and the output waveguide ofsaid third interferometer optical switch is used as the second outputport of said optical circuit.
 90. A variable optical attenuatorcomprising an optical circuit having a plurality of optical variableattenuates as defined in claim 43 connected in cascade, wherein a firstinterferometer optical switch having two output waveguides; one of thesaid output waveguides is connected to the input waveguide of a secondinterferometer optical switch; the other output waveguide of said firstinterferometer optical switch is connected to the input waveguide of athird interferometer optical switch; the input waveguide of said firstinterferometer optical switch is used as the input port of said opticalcircuit; the output waveguide of said second interferometer opticalswitch is used as the first output port of said optical circuit; and theoutput waveguide of said third interferometer optical switch is used asthe second output port of said optical circuit.
 91. An interferometeroptical switch using at least one interferometer optical switch asdefined in claim 1 to configure an optical switch with M inputs (M:natural number) and N outputs (N: natural number).
 92. A variableoptical attenuator using at least one variable optical attenuator asdefined in claim 43 to configure an optical switch with M inputs (M:natural number) and N outputs (N: natural number).
 93. Theinterferometer optical switch as claimed in claim 1, wherein saidoptical coupler consists of a directional coupler including two opticalwaveguides placed side by side in close proximity.
 94. The variableoptical attenuator as claimed in any one of claims claim 43, whereinsaid optical coupler consists of a directional coupler including twooptical waveguides placed side by side in close proximity.
 95. Theinterferometer optical switch as claimed in claim 1, wherein said phaseshifter consists of a thin film heater formed on the optical waveguide.96. The variable optical attenuator as claimed in claim 43, wherein saidphase shifter consists of a thin film heater formed on the opticalwaveguide.
 97. The interferometer optical switch as claimed in claim 1,wherein said phase shifter consists of a thin film heater formed on theoptical waveguide, and an adiabatic groove is formed near said thin filmheater.
 98. The variable optical attenuator as claimed in claim 43,wherein said phase shifter consists of a thin film heater formed on theoptical waveguide, and an adiabatic groove is formed near said thin filmheater.
 99. The interferometer optical switch as claimed in claim 1,wherein said optical waveguide circuit is made of a silica-based glassoptical waveguide.
 100. The variable optical attenuator as claimed inclaim 43, wherein said optical waveguide circuit is made of asilica-based glass optical waveguide.
 101. The interferometer opticalswitch as claimed in claim 1, wherein said interferometer optical switchhas birefringent index adjustment means on its optical waveguide, orundergoes adjustment of a birefringent index.
 102. The variable opticalattenuator as claimed in claim 43, wherein said variable opticalattenuator has birefringent index adjustment means on its opticalwaveguide, or undergoes adjustment of a birefringent index.
 103. Anoptical module comprising a module including within it an interferometeroptical switch as defined in claim 1, and optical fibers that are heldby said module for inputting and outputting a signal to and from saidinterferometer optical switch.
 104. An optical module comprising amodule including within it a variable optical attenuator as defined inclaim 43, and optical fibers that are held by said module for inputtingand outputting a signal to and from said variable attenuator.